The compound [Pt(Et 2 dien)Cl]Cl, where Et 2 dien = N,N-diethyldiethylenetriamine, has been synthesized and reacted with N-acetylmethionine (N-AcMet) and guanosine 5'-monophosphate (5'-GMP). Reaction with 5'-GMP at pH 4 leads to formation of [Pt(Et 2 dien)(5'-GMP)] + , as expected. Reaction with N-AcMet occurs more rapidly than with 5'-GMP; when the reaction with N-AcMet occurs at low pH values (~2), one set of resonances is observed initially but a second set of resonances appears over several days as the initial set decreases. The initial set of resonances disappears slowly (several days) at pH 2 but disappears much faster (minutes to hours) if the pH is 4 or higher; the initial resonances do not reappear even if the pH is lowered. Reaction of the [Pt(Et 2 dien)Cl] + with a mixture of N-AcMet and 5'-GMP at pH 4 leads to formation of a unique set of NMR resonances that does not correspond to the resonances of either the 5'-GMP or the N-AcMet products; mass spectrometry identifies a new product corresponding to [Pt(Et 2 dien)(N-AcMet)(5'-GMP)], in which the Et 2 dien ligand is bidentate. Such a complex is also observed if 5'-GMP is added to a sample of [Pt(Et 2 dien)(N-AcMet)] + that has been kept at low pH, but it is not observed when N-AcMet is added to a sample of [Pt(Et 2 dien)(5'-GMP)] +. We propose that the additional set of resonances observed for [Pt(Et 2 dien)(N-AcMet)] + at low pH is due to a less stable conformation of the complex and the addition of 5'-GMP when this less stable conformation exists leads to displacement of one of the nitrogen atoms.