An LKB flow microcalorimeter has been used to determine excess enthalpies, H(E), of mixtures of thiaalkanes, dithiaalkanes and polythiaalkanes + n-alkanes or + cyclohexane. These data, the previously measured excess Gibbs energies, G(E), and the data available in the literature on G(E), H(E) and activity coefficients at infinite dilution, gamma(i)infinity, are examined on the basis of the DISQUAC group contribution model. The interaction parameters, quasi-chemical and dispersive, depend on the environment of the S atom. There is a clear evidence for the S...S proximity effect, which produces a regular decrease in both the quasi-chemical and the dispersive interaction parameters. The model consistently describes the excess functions G(E) and H(E) of the investigated thiaalkanes + n-alkanes or cyclohexane mixtures. Characteristic deviations between predicted and experimental gamma(i)infinity data are observed in mixtures containing molecules of widely different sizes. The present study confirms the conclusions of other authors that the replacement of one or several CH2 groups in n-alkanes by S atoms does not break the "orientational order" related to molecular conformation.