Spring and damper characteristics determine to a large extent the ride quality and handling of a vehicle. Since the requirements for good ride and good handling are conflicting, adjustable suspension elements are developed. In this study a two-state semi-active hydro-pneumatic spring, in conjunction with a two-state semi-active hydraulic damper is investigated. A mathematical model of the spring/damper system is developed and verified with measured data. Two types of tests were performed on a prototype spring/damper unit, namely characterisation tests and single degree of freedom tests. The characterisation tests included characterising the hydro-pneumatic spring, the hydraulic damper, as well as the hydraulic valves in terms of valve response times. For the single degree of freedom tests, the step response, random input response and sine sweep response were determined. Simulation models of the characterisation setup, as well as the single degree of freedom setup were constructed in Matlab Simulink. A real gas, thermal time constant model was used for modelling the hydro-pneumatic spring, while a look-up table was used for the damper characteristics. A hydraulic flow model was developed from first principles and first order valve dynamics were also included in the models. Good correlation was obtained between measured and simulated data for the characteristation tests, as well as the single degree of freedom tests. The spring/damper model can be incorporated into a full 3D vehicle model in order to predict the ride and handling of a vehicle fitted with such a system.