A study of the hydroformylation of propene, but-1-ene, cis-but-2-ene, pent-1-ene, pent-2-enes, and 4-methyl-pent-1-ene has shown that the CO partial pressure has a remarkable influence on the isomeric composition of the aldehydes formed. In the straight-chain olefins the percentage of aldehyde arising by the introduction of the formyl group on the terminal carbon atom of the olefin chain increases as the CO pressure is increased, at pco <40 atm. Also the extent of isomerisation of the olefinic substrate is remarkably dependent on the CO partial pressure. The results have been explained on the basis of the existence of at least two catalytic species, the concentration of which depends on the CO partial pressure.