The RP-HPLC capacity factors (k') of a series of substituted biphenyls were determined on a C18column with methanol/water as the mobile phase. A linear relationship was found between log k' and the volume fraction of methanol (ϕ) for each of the 25 tested compounds with the correlation coefficient > 0.99. High correlation was found between log k'wand S, which are intercept and slope of plot of log k' vs. ϕ, respectively. The values of S and log k'wwere quantitatively correlated with the solvatochromic parameters of the solutes and two statistically significant equations were established which showed that solute volume and hydrogen bond basicity were of major importance in influencing the retention of these studied compounds. The obtained log k'wwas chosen as a measure of the solute hydrophobicity and will be used for the on-going Quantitative Structure-Activity Relationship (QSAR) studies of the substituted biphenyls.