Synthesis, characterization and in vivo anti-tumour effects of a series of organotin(IV) derivatives of N-maleoylglycine are reported. The ligand molecule appear to be bound to the tin atoms through the carboxyl oxygen atom. Since the compounds were not obtained as single crystals, various experimental techniques were used to characterize them and determine their structures. The results obtained by multinuclear magnetic resonance ( 1 H, 13 C and 119 Sn), fourier transform IR and 119m Sn M ΓΆssbauer spectroscopy and mass spectrometric analyses suggest a hexacoordination for the diorganotin(IV) carboxylates. The triorganotin(IV) carboxylates exist as pentacoordinated trigonal bipyramidal complexes in the solid state and tetrahedral ones in solution. In vivo toxicity profiles in mice and antitumour activities in tumour-bearing (colon 26A) mice were obtained for the title organotin(IV) carboxylates synthesized. At their maximum tolerated doses, no antitumour effect was observed for compounds 1, 2, 5 and 6; however, compounds 3 and 4 showed antitumour activity after single dose administration.