Abstract The aqueous chemistry of monoalkyltin trihalides is not yet fully understood. This work has elucidated, by 1H and 119Sn NMR spectroscopy, the hydrolysis products present in aqueous solutions of monomethyl- and monobutyl-tin trichloride containing various mole ratios of NaOH. The initial product upon dissolution of RSnCl 3 in water (0.5 M) is RSn(OH)Cl 2·2H 2O. At a mole ratio of RSnCl 3:NaOH of 1:1–2, two additional species are formed, RSn(OH) 2Cl·2H 2O and [RSn(OH)(H 2O) 4] 2+. At a mole ratio of 1:3 short chain oligomers are produced, containing SnOSn linkages and pendant OH groups. At a mole ratio of 1:4, dehydration occurs between oligomers to form a cross-linked or branched polymer. At a ratio of 1:6 or above, depolymerisation has occurred to afford a monomer RSn(OH) 3·2H 2O. In the case of RMe the final hydrolysis product is unstable and undergoes disproportionation to yield R 2Sn(OH) 2·2H 2O and ultimately Sn(OH) 6 2−.