The experimentally known dependence in RP-HPLC of the retention factor k′ on octanol/water partition coefficient (Kow) has been examined based on solvophobic theory. The result showed that the dependence provides a means for the evaluation of phase ratio (Φ) of RP-HPLC columns, and of the equilibrium constant for a given compound and mobile phase. Using this theory, the phase ratio was evaluated for a set of seven different C18 columns (five having fully porous particles and two core–shell particles), and the equilibrium constants were calculated for four homologous series of compounds in two mobile phase systems. One mobile phase was methanol/aqueous solution of 0.1% H3PO4, and the other was acetonitrile/aqueous solution of 0.1% H3PO4. Besides providing the values for Φ for the evaluated columns, the results of the study indicated that for a specific composition of the mobile phase and for a given compound displaying only hydrophobic interactions, the equilibrium constant K(X) for different C-18 columns is basically the same. The data were further used to provide guidance in the selection of a chromatographic column for a specific separation based on Kow values and chemical structure of the analytes. The study indicated that the separation of compounds with identical polar groups (or no polar groups) and with very close values for the Kow cannot be achieved based only on hydrophobic interactions that dominate the separation on RP-type columns. Only column that displays polar interactions may provide a solution to such separations. For hydrocarbons with close Kow values, the separation cannot be achieved even on columns with some polarity. On the other hand, even compounds with equal Kow values, but with different functionalities can be separated on RP-HPLC columns without involving polar interactions. The compounds with different Kow values are expected to be easily separated on RP-HPLC columns.