The modelling of the condensation of gaseous mixtures containing a non-condensing species is discussed. The model selected is applied to the countercurrent partial condensation of light hydrocarbons (methane, ethylene, ethane and propylene) in the presence of hydrogen. Calculations are performed to study the effect of Hz upon the equilibrium constants of other components, and also, to determine the extent to which the physical solubility of H-2 may affect the process. The simulations also demonstrate the sensitivity of the system to certain operating and geometric parameters and show the relative importance of these parameters.