Vapor pressures of binary mixtures of methylisocyanate with hexane and benzene in the temperature range of -5-degrees-C to 200-degrees-C have been measured. There was no indication of thermal instability or polymerisation within that range. Based on the experimental VLE data, UNIFAC interaction parameters have been correlated for the new main group of isocyanate. The parameters allow good prediction of VLE from different binary systems composed of molecules with the same functional groups. In addition, literature data of binary mixtures were taken to calculate interaction parameters of chlorine-substituted aliphatic hydrocarbon-groups with the isocyanate group.