Hydridocarbonyltris(triphenylphosphine)rhodium(I), RhH(CO)(PPh3)3, in benzene solutions is an homogeneous catalyst for the rapid isomerisation of alk-1-enes. Rate studies have been made with pent-1-ene, which gives a mixture of cis- and trans-pent-2-ene, the former predominating initially. The isomerisation of cis- to trans-pent-2-ene was slower than the conversion of pent-1-ene but still relatively important. It was shown that during the hydrogenation of pent-1-ene at 1 atm./27° catalysed by RhH(CO)(PPh3)3 the isomerisation to cis- and trans-pent-2-enes occurs at approximately the same rate as hydrogenation.