The rate of membrane permeation of molecules of the protein, lysozyme, dissolved in aqueous crystallization media depends on the square root of the ionic strength. This dependence can be explained by assuming that there is an equilibrium between lysozyme monomers and dimers. Using electrolyte theory and the hypothesis that the dimerization equilibrium involves exchange of H+ with the buffer, we calculate that a dimer in aqueous NaCl buffered to pH 4.0 at 20°C has a charge equal to +18.6 |e|, where e is the electron charge. We find further that the standard free energy of formation of the dimer from monomers is 173 kJ mol-1, while the equilibrium constant for dimer formation is 1.2 × 10-31. Although these values imply that dimers are not prevelant under standard conditions, we find that the mass action effects of H+ exchange with the buffer and the stablizing effects of the net electrostatic interaction of both monomers and dimers with Na+ and Cl- ions enhances the presence of dimers under the conditions ordinarily encountered in crystal growth experiments. This information is of importance in understanding the mechanism of lysozyme crystallization, because the formation of dimers is usually considered as the first step in the nucleation process that presumably precedes the appearance of the crystals.