The retention constants (RM) of a series of estrane and secoestrane derivatives are experimentally determined on C18-modified silica gel layers with methanol-water mobile phases of various concentrations. The slopes (m) and intercepts (RMW) of the linear relationships between RM and the volume fraction of methanol are calculated. Both constants increase when the retention of compounds increases, and there is a linear dependence between them indicating their additivity; they represent the sums of particular retention contributions of skeleton and substituents. The contributions, particularly the retention fragmental constants, are calculated by combining the linear relationships RM/log P, RM/m, and RMW/log P. The log P values of the compounds and skeleton were calculated using Rekker's fragmental constants.