New triamine-type platinum(II) complexes, [PtCl(amine)(diamine)]+(amine = 2-aminoethanol (ae-OH) or 2-methoxyethylamine (ae-OMe); diamine = (NH3)2, ethylenediamine (en),N,N′-dibenzylethylenediamine (dibenzylen) or 1,2-diphenylethylenediamine (stien)) were synthesized and their reactions with 5′-guanosine monophosphate (5′-GMP) were studied through1H NMR measurements. The rate constants for reactions of stien complexes are half those for the en complexes. The dibenzylen complex has lower reactivity because of the bulky benzyl groups. The resulting order of reactivities is en > stien > (NH3)2≥ dibenzylen. In comparison of monoamine ligands, the complexes of 2-methoxyethylamine have higher reactivities than those of 2-aminoethanol.