The counterions X in cis-[Pt(NH3)(2)(9-MeA-N7)(9-EtGH-N7)]X(2) (1) determine, in the solid state, which of the possible orientations the two purine nucleobases display. With X = PF6-, the right-handed helicoidal head-head conformation R2 encountered in double-stranded oligonucleotide adducts of cis-Pr(NH3)(2)(2+) is realized, together with its enantiomeric counterpart L2. 1 therefore provides a model of the d(ApC) intrastrand adduct of cisplatin with double-stranded DNA. H-bond formation between the platinated guanine in 1 and l-methylcytosine in DMSO-d(6) is not strictly analogous to that between the Watson-Crick pair of free 9-EtGH and 1-MeC.