The synthesis and X-ray structure (as the tetrahydrate) of the platinum(IV) complex trans, trans, trans[Pt(N-3)(2)(OH)(2)(NH3)(2)] 3 are described and its photochemistry and photobiology are compared with those of the cis isomer cis,trans,cis-[Pt(N-3)(2)(OH)(2)(NH3)(2)] 4. Complexes 4 and 3 are potential precursors of the anticancer drug cisplatin and its inactive trans isomer transplatin, respectively. The trans complex 3 is octahedral, contains almost linear azide ligands, and adopts a layer structure with extensive inter-molecular hydrogen bonding. The intense azide-to-platinum(iv) charge-transfer band of complex 3 (285 nm; epsilon = 19 500 m(-1) cm(-1)) is more intense and bathochromically shifted relative to that of the cis isomer 4. In contrast to transplatin, complex 3 rapidly formed a platinum(n) bis(5'-guanosine monophosphate) (5'-GMP) adduct when irradiated with UVA light, and did not react in the dark. Complexes 3 and 4 were non-toxic to human skin cells (keratinocytes) in the dark, but were as cytotoxic as cisplatin on irradiation for a short time (50 min). Damage to the DNA of these cells was detected by using the "comet" assay. Both trans- and cis-diammine platinum(iv) diazide complexes therefore have potential as photochemotherapeutic agents.