The Wiener index W was defined originally for saturated hydrocarbons. In this work the definition of W was extended for unsaturated hydrocarbons: W = Ws + Wd + Wt + Wa, where s, d, t, and a refer to the contributions of single, double, triple and aromatic bonds, respectively. Good correlation (r = 0.95) could be demonstrated between the logarithms of n‐octanol‐water partition coefficients of the molecules and the components Ws, Wd, Wt and Wa. Copyright © 1990 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim