Deuterium labelling shows that the action of heat upon trans-[Pt(C2H5) Br (PEt3)2] gives ethylene and trans-[PtHBr(PEt3)2] in which the hydridic hydrogen orginates from both the α and β-carbon atoms of the ethyl group. The position of equilibrium in the reversible reaction between olefins and platinum hydrides depends both upon the nature of the olefin and the group trans to the hydride. In reactions of higher olefins with trans-[PtHCl(PEt3)2] and of alkyl Grignard reagents with cis- and trans-[PtCl2(PEt3)2], n-alkyl-platinum derivatives are formed more readily than isoalkyl.