We have studied the retention indices of 127 volatile substances on a C-78-paraffin and on its seven nearly isochor and isomorphous polar derivatives over a temperature range of 90-210 A degrees C. The retention index of a substance on the C-78 paraffin has been considered as the standard. The additional retention on the polar derivative was given by the difference of its retention index on the polar solvent and on the C-78-paraffin. Statistical analyses of the additional retention have shown that with respect to retention, the seven polar solvents can be classified into three groups: Type I: TTF (tetrakistrifluoromethyl), MTF (monotrifluoromethyl), Type II: PCN (primary cyano), PSH (primary thiol) and Type III: TMO (tetramethoxy), SOH (secondary alcohol) and POH (primary alcohol). It is shown that these three types are best represented by the solvents TTF, PCN and TMO. PSH (primary thiol) is aligned with PCN at temperatures up to about 150 A degrees C, but is similar to TMO at 210 A degrees C.