Using our recently derived group contribution solvation (GCS) model, we have developed a predictive model for the octanol-water partition coefficient (K-OW), the GCSKOW model. In this model K-OW is calculated from two molecular structure parameters, which take into account the size and shape effects, and one energy parameter that determines the attractive interactions between the solute and the solvent. On the basis of quantum mechanical studies, we found that for organic solutes with a single strong functional group, all these parameters can be obtained in a group contribution manner. Consequently, we present a database here for various functional group contributions in this new, easy-to-use model. The root-mean-square deviation of the predicted log K-OW from the GCSKOW model for 226 solutes is found to be 0.14 (which corresponds to 38% in K-OW), which is considerably less than those from the methods of Hansch and Leo (0.18 in log K-OW or 51%), KOW-UNIFAC (0.21 or 62%), and LSER (0.23 or 71%).