A nonlinear model equation based on tissue composition (a content of lipids, proteins, and water) for the tissue-blood partition coefficients of compounds was further developed in this paper to account for the neutral and ionized forms of the compounds. The nonlinear model equation may be considered as the expressive form of the Hansch equation in nonlinear spaces (or multiphase system). Based on this model our nonlinear regression analysis for neutral and ionized compounds partitioning into kidney, brain, muscle, lung, liver, heart, and fat resulted in equations with high fitting power (training set: n=201, r=0.905, s=0.291, Q=0.890) and strong predictive power (test set: n=64, r=0.906, s=0.247). These results showed that the equilibrium distribution of a compound in a variety of tissues is essentially the equilibrium distribution of the compound in tissue (chemical) compositions. Neutral and ionic forms of a compound as well as in different tissue (chemical) compositions have a different mechanism of action in vivo.