Kinetics for the reactions of [1] [PtIV2Me8(&mgr; - SMe2)2] + 2 Me2Skf<-->kd2[PtIVMe4(SMe2)2] [2] [PtII2Me4(&mgr; - SMe2)2] + 2 Me2Skf--> 2[PtIIMe2(SMe2)2]and [3] [PtIV2Me8(&mgr; - SMe2)2] + 2NN --> 2 [PtIVMe4(NN)]+2Me2Swhere NN = bipy or 4,4'-Me2-bipy, have been studied at various temperatures and pressures. Reaction [3] was shown to consist of the rapid conversion of the dimer to a monomer and the much slower subsequent substitution of the dimethyl sulfide with bidentate ligand NN: [PtIV2Me8(&mgr; - SMe2)2] + 2 Me2S <--> 2 [PtIVMe4(SMe2)2] and [PtIVMe4(SMe2)2] + NN --> [PtIVMe4(NN)] + 2 Me2S The rate constants and activation parameters for the reactions are as follows:kf= 3.16 ± 0.06 M-1s-1(25°C), &Dgr;Hdoubledaggerf= 51.8 ± 1.7 kJ mol-1, &Dgr;Sdoubledaggerf= -61.0 ± 5.8 J mol-1K-1,kd= 1.18 ± 0.22 M-1s-1(25°C), &Dgr;Hdoubledaggerd= 65 ± 22 kJ mol-1, &Dgr;Sdoubledaggerd= -26 ± 73 J mol-1K-1for reaction [1] inn-hexane;kf= 6.68 ± 0.06 M-1s-1(25°C), &Dgr;Hdoubledaggerf= 58.0 ± 3.1 kJ mol-1, &Dgr;Sdoubledaggerf= -34.5 ± 10.5 J mol-1K-1, &Dgr;Vdoubledaggerf= -10.7 ± 1.3 cm3mol-1for reaction [2] in dichloromethane;k2= (7.09 ± 1.89) x 10-4M-1s-1, &Dgr;Hdoubledagger2= 95 ± 21 kJ mol-1, and &Dgr;Sdoubledagger2= 18 ± 70 J mol-1K-1, &Dgr;Vdoubledagger2= 9 ± 9 cm3mol-1for reaction [3] with bipy, andk1= (1.10 ± 0.10) x 10-2s-1,k3/k-1= (4.33 ± 0.30) x 10-2, andk2= (6.09 ± 1.35) x 10-4M-1s-1for reaction [3] with 4,4'-Me2-bipy. It was shown that reactions [1] and [2] proceed nucleophilically without any intermediates, and reaction [3] proceeds through a mainlyk2path for NN = bipy and through bothk1andk2paths for NN = 4,4'-Me2-bipy, without appreciable participation of the direct reaction between the dimer and NN as shown by the following reaction scheme.Key words: octamethylplatinum(IV) dimer, tetramethylplatinum(II) dimer, reaction mechanism, high-pressure kinetics.