The thermal decompositions of complexes cis-[PtR2L2][R = Ph or C6H4Me-4; L2=(PPh3)2, {P(C6H4Me-4)3}2, (PMePh2)2, Ph2PCH2PPh2, Ph2PCH2CH2PPh2, or Me2PCH2CH2PMe2] in toluene solution have been investigated. Their stability to thermolysis varies markedly with L2, and only complexes of monotertiary phosphines are labile at 60 °C. Reaction occurs via primary concerted unimolecular reductive elimination of biaryl, and conforms to a first-order kinetic rate law. Concurrent but independent secondary decomposition of [PtL2] generates both arene and biaryl as minor products. The elimination of 4,4′-bitolyl from cis-[Pt(C6H4Me-4)2(PPh3)2] exhibits a pronounced negative entropy of activation, suggestive of a conformationally restricted transition state. The inclusion of free PPh3 suppresses secondary processes and accelerates primary decay. Although itself inert under these conditions, [PtR2(Ph2PCH2PPh2)] also eliminates biaryl in a concerted and kinetically first-order manner when allowed to react in the presence of 10 equivs. of Ph2PCH2PPh2. These observations provide further evidence for the facilitation of reductive elimination by added nucleophiles.