A series of diplatinum(III) complexes derived from cis-(NH3)2PtII and the model nucleobase 1-methylcytosine (1-MeC) has been prepared and X-ray structurally characterized, all of which contain two anionic base ligands (1-MeC−) in a head–tail (ht) arrangement: ht-cis-[(ONO2)(NH3)2Pt(1-MeC−-N3,N4)2Pt(NH3)2(ONO2)](NO3)2·HNO3·3H2O (2b), ht-cis-[(NO2) (NH3)2 Pt(1-MeC−-N3,N4)2Pt(NH3)2(OH2)](ClO4)3·3.5H2O (3), ht-cis-[(OH2)(NH3)2Pt(1-MeC−-N3,N4)2Pt(NH3)2(OH2)](ClO4)4·H2O (4b), and ht-cis-[(9-EtGH-N7)(NH3)2Pt(1-MeC−-N3,N4)2Pt (NH3)2(9-EtGH-N7)](NO3)4·9H2O (7b) (9-EtGH=9-ethylguanine). Several other compounds, differing in the nature of the axial ligands, have been isolated and or observed in solution by 1H and 195Pt NMR spectroscopy. The chemistry of these diplatinum(III) compounds is dominated by facile substitution reactions of the axial ligands. Of particular interest in this context is the ready reaction of 2b or 3 with guanine nucleobases. Since similar compounds are not obtained with any of the other common nucleobases, 2b and 3 can be considered guanine-specific chemical probes.