The hydrolysis of methyltin(IV) cations at fairly high concentrations was investigated to evaluate the formation of polynuclear species in aqueous solution. The hydrolysis of monomethyltin(IV), dimethyltin(IV), and trimethyltin(IV) was studied by potentiometry at T = 298.15 K and at I = 1 mol.L-1 in NaNO3 aqueous solutions. The results obtained gave evidence for the formation of the following polynuclear species, in addition to the mononuclear species already reported, which were also considered in the models proposed for the three systems investigated: [(CH3)Sn(OH)(3)](0), [(CH3)Sn(OH)(4)](-), [((CH3)Sn)(2)(OH)(4)](2+), [((CH3)Sn)(2) (OH)(5)](+), [((CH3)Sn)(2)(OH)(7)](-), [((CH3)Sn)(3)(OH)(5)](4+), a(CH3)Sn)(3)(OH)(7)](2+), [((CH3)Sn)(5)(OH)(9)](6+), and [((CH3)Sn)(5) (OH)(11)](4+) for the monomethyltin(IV) system; [( (CH3)(2)Sn) (OH)](+), a(CH3)(2)Sn)(OH)(2)](0), [((CH3)(2)Sn)(OH)(3)](-), [((CH3)(2)Sn)(2) (OH)(2)](2+), [((CH3) Sn-2)(2)(OH)(3)](+), [((CH3)(2)Sn)(3)(OH)(4)](2+), and [((CH3)(2)Sn)(4)(OH)(6)](2+) for the dimethyltin(IV) system; [((CH3)(3)Sn) (OH)](0), R(CH3)(3)Sn)(OH)(2)](-), [((CH3)(3)Sn)(2)(OH)](+), (CH3)(3)Sn)(2)(OH)(2)](0), and [((CH3)(3)Sn)(2)(OH)(4)](2-) for the trimethyltin(IV) system. The formation constants beta(OH)(pq) (relative to the equilibrium p(CH3)(n)Sn(4-n)+ + qOH(-) = ((CH3)(n)Sn)(p)(OH)(4)((p(4-n)-q))) can be expressed as a function of the cation charge z (z = 4 - n) by the general equation: log beta(OH)(pq)/(p+q) = 1 + (2.50 +/- 0.05)z, that allows the calculation of rough values of beta(OH)(pq) for any (p,q) couple. The reliability of the proposed speciation models is discussed. As expected for a tricharged cation, by increasing the concentration of (CH3)Sn3+, the yield of polynuclear species becomes predominant; this behavior is less marked for (CH3)(2)Sn2+ and (CH3)(3)Sn+.