A thermodynamic equation is used to analyze the retention behavior of phenolic solutes at low modifier concentrations on alkenyl and diol chemically modified silica packings. These materials were prepared by attaching alkenyl and diol ligands with four, six, eight and ten carbon atoms to the surface of LiChrosorb Si-60 silica. The thermodynamic analysis of the capacity ratios of various phenolic solutes measured at different concentrations of simple aliphatic alkohols (used as modifiers) in n-hexane have provided information about solute and solvent interactions with the alkenyl and diol modified silicas. This analysis shows that the solute-alcohol competitive interaction for the alkenyl bonded phases changes significantly with chain length, whereas this effect is not observed for the diol phases. Also, the influence of polarity and geometric structure of functional groups on solute retention have been examined.