In carrying out the oxo process utilizing a Co catalyst, considerable losses of catalyst substance occur in the formation of Co carbonyl and its subsequent removal from the reaction zone. This disadvantage is now overcome by utilizing 2 fixed-bed reaction zones in series, each reactor being packed with a supported Co-type catalyst. In operation, olefins are fed to the initial reactor through which synthesis gas is preferably recycled. The liquid oxo product thus obtained is passed to a 2d reactor through which H2 is recycled. Co carried from the initial reactor as Co carbonyl is deposited on the catalyst support in the 2d reactor. After the Co in the 1st reaction zone is depleted, the flow of the streams is reversed thus making it possible to operate continuously with a fixed-bed catalyst with substantially little loss of Co from the system. Small Co losses may be compenated for by introducing Co carbonyl or a Co salt of a fatty acid into the olefin feed. Also describes procedure whereby a single reactor may be employed for both the oxo and hydrogenation steps.