The oxidative addition of the salt [{SC(NMe(2))(2)}(2)]Cl-2 . 2H(2)O (1), the disulfide-like dimerized form of 1,1,3,3- tetramethylthiourea (tmtu), to Pt(II) chloro am(m)ine compounds is described. Oxidation of the [PtCl3(NH3)](-) anion with 1 in methanol yields cis-[PtCl4(NH3)L] (2; L = tmtu) as the result of the trans addition of one tmtu and one chloro ligand. The same mode of oxidation is found in reactions of 1 with [PtCl(dien)](+) (dien = diethylenetriamine) and trans-[PtCl2(NH3)2]. In these cases, however, the oxidation is followed by (light-independent) cis, trans isomerizations, giving trans,mer-[PtCl2(dien)L]Cl-2 (4) and fac-[PtCl3(NH3)(2)L]Cl . O.5MeOH (6), respectively. The single-crystal X-ray structures of 2 and trans,mer-[PtCl2(dien)L](BF4)(2) (4a) have been determined. 2 : monoclinic, space group P2(1)/n, a = 6.280(1) Angstrom, b = 13.221(3) Angstrom, c = 16.575(2) Angstrom, beta = 96.45(1)degrees, Z = 4. 4a : monoclinic, space group C2/m, a = 21.093(5) Angstrom, b = 8.9411(9) Angstrom, c = 14.208(2) Angstrom, beta = 124.65(2)degrees, Z = 4. The tmtu ligands are S-bound. In 2 a pronounced trans influence of the S-donor ligand on the Pt-Cl bond (2.370(1) Angstrom) trans to sulfur is observed. The unusual acidity of the Pt(IV) complexes exhibiting tmtu coordination trans to chloride is attributed to hydrolysis of the labilized Pt-Cl-trans bond, which is supported by ion sensitive electrode measurements. An upfield shift of the Pt-195 resonances is found on changing the ligand combination from NCl4S (2) to N3Cl2S (4). This order correlates with the trans influences of the ligands : tmtu > am(m)ine > chloride. The cytotoxicity of 2 and 6 in L1210 cell lines is reported and discussed in terms of a possible mechanism of action of the compounds in vivo. It is suggested that tmtu may act as a lipophilic carrier ligand and therefore enhance the cellular uptake of the new potential Pt(IV) drugs.