The crystal structure of [Pt(diethylenetriamine)(guanosine)](ClO4)2has been determined by X-ray diffraction. The crystals are orthorhombic, space groupP212121, witha = 12.486(6),b = 13.444(7),c = 14.678(11) Å, andZ = 4. The structure was refined by block-diagonal least-squares analysis to a conventionalRfactor of 0.050 and a weightedRw = 0.045.The coordination around the platinum atom is square planar. Guanosine is bonded to platinum through N(7). The purine planar ring makes an angle of 62.7° with the platinum coordination plane. The structure is stabilized by hydrogen bonding.