Values of the substituent entropy constants σ_s^o for monosubstituted methane and benzene derivatives were determined from the standard entropy of the 3rd law of thermodynamics. This parameter is orthogonal to σ_t or σ_π, and seems preferable to π, E_s, MR, M_w, P_r and V_w. Two examples of QSAR are successfully analysed in terms of σ_s^o.