At 50°C, isomerization of pent-1-ene is catalyzed by solutions of (i), Cl3(Ph3P)3Rh(III) in chloroform, in chloroform + alcohol mixtures, and in benzene, (ii), α-, and β-HCl2(Ph3P)3Rh(III) in benzene, and (iii), HCl(Ph3P)3Ru(II) in benzene. Cl2(Ph3P)3Ru(II) in benzene shows low activity. Characteristics of conversion-time curves are reported for each system. Whether or not reactions showed induction periods depended upon the nature of the complex, solvent composition, and the length of time for which solutions were allowed to stand before the addition of pent-1-ene. Some systems were very selective for the formation of pent-2-ene in the cis-form (cis : trans 18 : 1), whereas others were not (cis : trans 1.5 : 1). Cis-trans isomerization of the pent-2-ene was very slow compared to the rate of conversion of pent-1-ene to pent-2-ene, except when the catalytic solutions consisted of Cl3(Ph3P)3Rh(III) dissolved in CHCl3+C2H5OH mixtures which contained a high proportion of ethanol. There is evidence that the catalytic species are formed by solvolysis of the original complexes. Isotopic tracer studies show that, when methanol is present, catalyzed hydrogen-atom exchange occurs between olefin and methanol. Catalysis by rhodium complexes is facilitated if the complex contains a hydrogen ligand, and moreover the stereochemistry of the hydrido-complex appears to be important. Two ruthenium complexes, Cl2(Ph3P)3Ru(II), HCl(Ph3P)3Ru(II), have been examined to establish whether a “vacant ligand site” constitutes a useful feature of the original complex. Reactions of ethylene, particularly of trans-dideuteroethylene, with solutions of these complexes at 50°C have been investigated; isotope redistribution shows that reaction proceeds via ethyl intermediates. This supports the proposal that pentene isomerization occurs via pentyl intermediates and that the catalytic species in solution are hydrides.