The influence of pressure on the upper critical solution temperature (UCST) as well as on the lower critical solution temperature (LCST) behaviour of the system methylcyclohexane (MCH)/polystyrene PS 405 was studied. Cloud-point pressures up to 120 bars were measured with a Cailletet apparatus. Cloud-point and spinodal pressures were obtained with a pressure pulse-induced critical scattering apparatus (PPICS) up to 800 bars. Also, some three-phase equilibria liquid-liquid-vapour were determined. The lower demixing temperature is found to decrease with increasing pressure. The upper demixing temperature increases with increasing pressure. The experimental data were described qualitatively with a simple G(E)-model. This G(E)-model is able to describe the dependence of the demixing temperature on the pressure. Also, the model is able to predict the merging of the regions of immiscibility that can occur in systems of methylcyclohexane/polystyrene.