Abstract Amino acid hydrophobicity parameters, G hp log P (partition coefficient) values, free energies of solution, G sol and hydration numbers, are well correlated by equations derived from the relationship O X = Aα X + Lσ IX + Sν X + I iX + H 1 n HX + H 2 n nX + b 0 where O is the quantity correlated; X denotes the amino acid side chain; α is a polarizability parameter; σ I , a localized electrical effect parameter; ν, a steric parameter; i, an indicator variable which accounts for an ionic X ; n H and n n the number of OH or NH bonds and of full nonbonding orbitals in X, respectively, and b 0 is the intercept. The equation is based on the assumption that Δ hp log P and ΔG sol are all functions of the difference in intermolecular forces between the amino acid and some medium, and the amino acid and water. The parameters were chosen to model the intermolecular forces of interest. Generally the most important factor is α x . This is followed by ν, i, and n H . Least important is σ I . ΔG sol depends on α, n H and n n . Hydration numbers depend on i, n H and n n . The hydrophobicity of amino acid side chains is the result of a preference for a nonpolar medium as a increases and for a polar medium as i, n H and σ I increase. It is quantitatively accounted for by the model, and no special “hydrophobic bond” need be involved. The results show that log P values for amino acids are composite quantities whose composition is variable.