The enantiomers of the asymmetric, chiral platinum(II) complex [PtCl 2(S-ahaz)] (S-ahaz ) 3(S)-aminohexahydroazepine) each form two stereoisomers on binding to GpG sequences of DNA: one in which the primary amine is directed toward the 5′ end of the DNA and one in which it is directed toward the 3′ end. Previous binding studies have revealed that the S-enantiomer forms the two stereoisomers in a 7:1 ratio while the R-enantiomer forms them in close to a 1:1 ratio. In an attempt to elucidate the reasons behind the stereoselectivity displayed by the S-enantiomer and to establish which isomer is formed in the greater amount, we report here its reaction with a 14-mer oligodeoxyribonucleotide having a single GpG site. The two stereoisomers that formed were separated using HPLC methods, and their integrities were confirmed by electrospray ionization mass spectrometry. The DNA duplex was formed by combination of each of the purified reaction products with the complementary strand of DNA. Identification of both of the stereoisomers was achieved using 2D NMR spectroscopy, which is the first time this has been achieved for an unsymmetric platinum complex bound to DNA. The minor stereoisomer, with the bulk of the ahaz ring directed toward the 3′ end of the platinated strand, induced considerable disruption to the 14-mer DNA duplex structure. The primary amine of the ahaz ligand was oriented toward the 3′ side of the duplex in the major isomer, giving a DNA structure that was less disrupted and was more akin to the structure of the DNA on binding of cisplatin to the same sequence.