Reaction of [PtCl 2 (SMe 2 ) 2 ] or K 2 PtCl 4 with two equivalents of the water soluble phosphine ligand, 1,3,5-triaza-7-phosphaadamantane (PTA), afforded crystals of {cis-[PtCl 2 (PTA) 2 ]} 2 .H 2 O, of which the solid state structure was determined by X-ray crystallography. This study describes the first crystallographic study of a cis PTA complex of Pt(II). It crystallizes in the triclinic space group P1 with two crystallographically independent Pt(II) moieties in the asymmetric unit which are linked by strong hydrogen bonds via a common water molecule. The title compound exhibits the shortest Pt-P bonds (avg 2.2263(14) 9) known to date. 3 1 P NMR shows that in aqueous solution an equilibrium exists between the cis bis isomer and [PtCl 3 - n (PTA) 1 + n ] (n = 2, 3) complexes.