As a result of the determination of retention indices (RI) of 11 organic compounds of various polarity in reversed phase (RP) HPLC using methanol as organic solvent in eluent, the coefficients of the dependence of RI vs. content of methanol, dRI/dС were determined. Formally, these parameters are equivalent to the temperature coefficients of gas chromatographic retention indices, β = dRI/dT. It is shown that the ranges of variations of these coefficients are rather large (from -2.0 up to +4.6 in the current work), which requires revealing the factors influencing their signs and absolute values. The correlations of coefficients dRI/dC with retention indices themselves, as well as with hydrophobicity factors, logP (the logarithms of partition coefficients in the system 1-octanol/water) seem to be negligible. The best correlations of dRI/dC were observed with such polarity measures of organic compounds as homologous increments of hydrophobicity factors (i _logP), and retention indices (i _RI). Such transformations of properties A as their homologous increments characterize different homologues despite of their locations within the homologous series. Thus, the parameters i _logP and i _RI appeared to be the measures of the polarity of the corresponding series. This approach permits us to reveal that the dependence of retention indices in RP HPLC on the content of methanol in eluent is determined just by the analytes' polarity and their hydrophobic – hydrophilic properties. The ratio dRI/dC> 0 is typical for non-polar analytes, while for polar compounds the negative signs of these coefficients are typical, dRI/dC< 0. Hence, determining even the sign of this coefficient provides the important information on the chemical origin of analytes.