The hydrolysis of monomethylmercury(II) was studied potentiometrically, in NaNO3, Na2SO4, and NaCl aqueous solution, in a wide range of ionic strengths (NaNO3, 0 less than or equal to I less than or equal to 3.25; Na2SO4, 0 less than or equal to I less than or equal to 1; NaCl, 0 less than or equal to I less than or equal to 3 mol dm(-3)) and at t = 25 degrees C. For the reaction (CH3)Hg+ = (CH3)Hg(OH)(o) + H+, we found log K-1 = -4.528 (I = 0 mol dm(-3)). The species [(CH3)Hg](2)(OH)(+) was also found, with log beta(2) = -2.15. Monomethylmercury(II) forms quite strong complexes with Cl-(log K = 5.45, I = 0 mol dm(-3)) and SO42-(log K = 2.64, I = 0 mol dm(-3)). The dependence on ionic strength of formation constants was considered by using a Debye-Huckel type equation. Hydrolysis and complex formation constants (at different ionic strengths) obtained were used to calculate the interaction parameters of Fitter equations.