Infinite dilution activity coefficients (gamma(13)(infinity)) for mixtures of alkanes (C-3-C-8) in alkylcyclohexanes (C-10-C-18), and alkenes in alkylcyclohexanes (C-10-C-18), are reported. These were determined using gas-liquid chromatography where the alkylcyclohexanes form the stationary phase. A lattice theory for liquids, which has been previously applied to hydrocarbon mixtures, was used to calculate interchange energies from the experimental results. This theory was also applied to existing (cyclohexane + alkane) and (alkane + nonadecylcyclohexane) data with these interchange energies compared with values determined from the gamma(13)(infinity) results reported in this work. There was good agreement (1 to 5 per cent) between predicted and experimental gamma(13)(infinity) results for (alkane + alkylcyclohexane) mixtures. However, this was found not to be the case for the (alkene + alkylcyclohexane) mixtures where the alkene segment -CH=CH2 was assumed to behave similarly to the -CH2-CH2-segment. The differences between experimental and theoretical gamma(13)(infinity) values for these types of mixtures were of the order of 30 per cent when using interchange energies derived from(alkane + alkylcyclohexane) mixtures.