Abstract The solvation equation log SP = c + rR 2 + sπ 2 H + aΣα 2 H + bΣβ 2 H + vV x has been applied to reversed-phase HPLC capacity factors, as log k', for solutes on a C 18 bonded phase, with various water-methanoi mobile phases, using data by Yamagami and Takao. Here. SP is a property for a series of solutes in a fixed solvent system, and the explanatory variables are solute descriptors as follows: R 2, is an excess molar refraction, π 2 H is the solute dipolarity/polarizability, Σα 2 H and Σβ 2 H are the solute overall or effective hydrogen-bond acidity and basicity, and V x, is the McGowan characteristic volume; c. r, s, a. h and v are constants. It is shown that the blend of factors that influence log k in any given system is not the same as that which influences log P oct. In particular, solute hydrogen-bond acidity considerably influences log k', but has no effect on log P oct. It follows that when log k' values are used to estimate log P oct. great care has to be taken to match the training set of solutes in the correlation equation, with the solutes for which log P oct is to be determined.