Reactions of 5′-guanosine monophosphate (5′-GMP) and N-acetylmethionine (N-ac-l-Met) with the mononucleobase compounds, cis-[PtCl(L)n(9-EtGH)]+ (L = NH3, 4-pic, n = 2; L = en, n = 1) in a 1:1 molar ratio have been studied in aqueous solutions at pH 7.3 using 1H and 195Pt NMR spectroscopy. There is a high kinetic preference for sulfur over nitrogen binding. These results are compared with the trans isomers. Based on low cytotoxicity and a high sulfur/nitrogen preference the cis isomers may also present suitable features for antiviral activity through interaction with specific proteins.