Infinite dilution activity coefficients gamma(13)(infinity) and Henry’s law constants H for mixtures of alkanes and alkenes ( (C-3 to C-6) in phenylalkanes (C-10 to C-19) are reported. These were determined using gas-liquid chromatography (g.l.c.) A lattice theory for liquids, which has previously been applied to these types of mixtures, was used to calculate interchange energies from the experimental data. These were found to differ from those reported by previous workers. Good agreement (between 1 per cent and 5 per cent) was found between predicted and experimental gamma(13)(infinity) results for (alkane + phenylalkane). However, this was found not to be the case for the (alkene + phenylalkane) when 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 + phenylalkane.