Thermodynamic and spectroscopic properties of the species formed by dimethyltin(IV) cation with L-cysteine (cys) were studied by potentiometric, calorimetric, UV and NMR investigations in aqueous solution. The resulting speciation model showed the formation of five complex species: (CH(3))(2)Sn(cys)H(+), (CH(3))(2)Sn(cys)(0), (CH(3))(2)Sn(cys)OH(-), (CH(3))(2)Sn(cys)(2)H(-), (CH(3))(2)Sn(cys)(2)(2-). The stability and the formation percentages, for the mononuclear mixed species in particular, are very high, in a wide pH range. Thermodynamic parameters indicate that the enthalpy values are exothermic and the enthalpic contribution to the stability is higher than entropic one. Individual UV spectra of cys and dimethyltin(IV)-cys species were calculated. Spectroscopic results of UV and (1)H NMR investigations fully confirm the speciation model. The structures calculated from NMR investigations show that all the species have an eq-(CH(3))(2)-tbp structure.