Many universities in South Afirca use alternative admissions tests together with results of the Grade 12 examinations for access of placement. These tests focus on academic literacy and mathematical skills and doe not provide information about proficiencies in disciplines other than mathematics. The implementation of new currculum for Grades 10-12 in South Africa generates a need to monitor preparedness of first-year students to align first-year curricula as closely as possible to the content of the new syllabi. The need for a tailor-made istrument to assess preparedness for chemistry at the tertiary level was prompted by the lack of suitable tests with the appropriate focus, depth and coverage for application in the South African context. This paper describes the development and evalution of a test instrument designed to assess and monitor baseline conceptual understanding in chemistry at the secondary-tertiary interface. The study was carried out with mainstream students at the university of Pretoria and Cape Town (N=513 and 258, respectively). Data analysed using the Rasch measurement model confirmed that the majority of students from both cohorts was reasonably well prepared for chemistry at the tertiary level. The use of the instrument for diagnostic purpose was demonstrated. It was possible to identify gaps in the assumed pre-knowledge of students who have qualified for admission to chemistry at a tertiary level at their respective institutions. Inadequate pre-knowledge was most noticeable in the areas of chemical reactions and electrochemistry. In addition, the instrument also shows promise as a placement tool within programmes. Its predictive ability compares well with non-South African placement tests and with Grade 12 mathematics performance, the latter is widely used for this purpose in South Africa.