The formation constants of the species formed in the systems H+ = dimethyltin(IV) + tyrosine, tryptophan, and phenylalanine as well as H+ + tyrosine, tryptophan, and phenylalanine have been determined in aqueous solution in the pH range (1.5 to 9.5) at a constant temperature (25 degrees C) and constant ionic strength (0.1 mol.dm(-3) NaClO4), using a combination of spectrophotometric and potentiometric techniques. The composition of the formed complexes were determined, and it was shown that dimethyltin(IV) forms three mononuclear 1:1 species with all the ligands, of the type MHL, ML, and MH-1L, where M and L represent metal ion and each amino acid, respectively. A complex distribution curve depending on the pH and metal ligand ratio is given, and influences of the ligands side chains on the stability constants of the formed species are discussed. H-1 NMR investigations in aqueous solution confirmed the species formation and led us to propose their probable structures.