The educational change brought about by the South African Constitution and the Language in Education policy has resulted in many parents choosing to send their children from pre-school to an English medium school, so that their children can become proficient in the English language. As a result, many learners are learning in a language that is not their mother tongue and are therefore struggling to acquire reading and writing skills. Although there are many misconceptions regarding the learning of a language, the underlying theory on how learners learn a second language through Cummins BICS and CALP theory is explored in this research. The main purpose of this study was to evaluate the effects of the Sheltered Instruction Observation Protocol (SIOP) on teaching English Second Language (ESL) learners in primary schools. The study further intended to determine the effectiveness of SIOP in the South African context. To facilitate the understanding of the problem statement of this study, an evaluation framework based on the Context- Input-Process-Product (CIPP) model of Stufflebeam’s model was applied. The research was carried out at one primary school who had predominantly ESL learners in the school and who had teachers trained in implementing SIOP. The main research question posed by this research is to determine the effectiveness of SIOP on teaching an ESL learner in a primary school in the South African context. The sub-questions of the research are to what extent does SIOP differ from traditional methods, how does SIOP affect teachers’ planning and instruction in the classroom; to what extent to the learners understand the content when the teachers implement SIOP and to what extent does SIOP contribute to the improvement of academic performance of ESL learners in content areas. To answer the research question, mixed methods was used to fit together the insights provided by qualitative and quantitative research into a workable solution and therefore make use of the pragmatic method as a system of philosophy. As qualitative data was collected first, followed by the analysis of the secondary data, the typology used for the research is the QUAL quan. Interviews and observations were carried out on teachers who are currently implementing SIOP using interview protocol and observation protocol and data obtained from the University of Pretoria was analysed to determine academic performance of ESL learners. Legitimation occurred at various stages of the research which included peer review and member checking, From the data collected through interviews and observations, the following themes emerged, namely (i) language focus; (ii) difference between SIOP and the traditional methods of teaching; (iii) planning and assessment; (iv) learner attitude towards SIOP and (v) Grade R influence. Although data collected from the University of Pretoria showed gains achieved, the methodology conducted in this research was not an experimental design. A paired ttest was used to determine gains made between the target school, comparison school and the whole sample. Therefore, tentative statements regarding the influence of SIOP on academic performance is thus made in this research. Although no experiment was undertaken for this research, the findings from the data show that the null hypothesis indicates that there is no significant difference between the scores of the baseline assessments and the follow-up scores. As the significant values (p-value) were less than .05, the null hypothesis can be rejected. As part of the analysis, the effect sizes were also calculated which indicates a large effect size for the total scores and reading scores while the phonics scale showed a medium effect and the mathematics scale showed a small effect size. Based on the results of the study, it was found that the programme needed some revisions to make better use of the existing opportunities. The following recommendations and suggestions are that an experiment design should be explored to ascertain actual contribution SIOP can make to an ESL class. SIOP should be implemented as a teacher development programme and possibly a module for teacher training at university level. Resources needs to be readily available for teachers to use which could contribute to the improvements of academic performance of ESL learners. Teachers should form planning teams with other teachers to share good practices and lastly adequate support is needed for the implementation of SIOP. The importance of this study is that it provides insight into and understanding of the role of implementing an intervention programme to improve academic performance of ESL learners. It also sheds light on the need for an intervention programme and the extent to which the programme can improve academic performance of ESL learners. The main contribution of this study to the body of knowledge was its insight into and understanding of a number of factors, categorised as inputs, process and outcomes, that had an effect of the implementation of the SIOP programme. The development of the conceptual framework was applied by the CIPP evaluation model, as a framework that illustrated the relationships and outcomes of the SIOP programme that had previously not been done. This study thus emphasises the importance of intensive research and proper planning of intervention programmes. The aim of this inquiry was to determine the effectiveness of a model of teaching that was developed in the USA. Throughout the research, the findings presented indicated that the Sheltered Instruction Observation Protocol would be a functional model to employ in South Africa’s education system and, if implemented correctly, would yield positive results. Therefore, this study was an initial step towards implementing a programme that would improve academic performance in schools, where the LoLT is different from the mother tongue. Although some valuable information was obtained, there is much scope for further research on how to evaluate the effects of SIOP on academic performance of ESL learners.