The action of strong acids on the complexes K[Pt(acac)2X](X = Cl, Br) gives a yellow “acid,” HPt(acac)2X (X = Cl, Br). The structure of this complex has been investigated by infrared and n.m.r. spectroscopy, considerable use of deuterated derivatives being made in assigning the spectra. It is suggested that in addition to conventional bidentate oxygen chelation of one of the acetylacetone groups, the other acetylacetone is bonded as a π-complex of the tautomeric enolic form of the acetylacetone molecule. The action of acid on the complex K[Pt(acac)3] has been shown to give two derivatives depending upon the acid employed. With hydrochloric or hydrobromic acid, a condensation of the two carbon-bonded acetylacetone groups occurs with the formation of a bicyclic dienyl ring structure which co-ordinates to the platinum to give the complex Pt(C10H14O3)X2(X = Cl, Br). The nature of the complex has been established by n.m.r. and infrared techniques and confirmed by X-rays. With sulphuric, phosphoric, or nitric acid, the complex K[Pt(acac)3] yields a polymer Pt(C5H7O2)2 isomeric with Pt(acac)2. Reaction of this with pyridine gives the adduct (py)2Pt(acac)2, which has been shown by infrared and n.m.r. techniques to be a planar PtII derivative with both acetylacetone groups carbon-bonded to the metal. The initial product is formulated as a polymeric adduct in which both acetylacetone groups are carbon-bonded to the platinum and the co-ordination number of four attained by bonding through some of the carbonyl groups of the carbon-bonded acetylacetone ligands.