Activation of cobalt preformer catalyst for oxo process

In the process for preparing oxo alcohols by the cobalt catalyzed hydroformylation of C.sub.2 to C.sub.17 linear or branched monoolefins with subsequent hydrogenation of the hydroformylation product, in which oxo process aqueous solutions of cobalt salts are converted to active hydrido cobalt carbonyl catalyst species in a preformer reactor, the preformer containing a preformer metal catalyst being a metal of Group IB or VIII of the Periodic Table, or a nonmetallic catalyst selected from the group consisting of activated carbon, ion exchange resins, silica alumina and zeolites, the improvement which comprises activating said preformer catalyst by treating it at a temperature of 120.degree. C. to 170.degree. C. and a pressure of about 13.78 MPa (2,000 psig) to 31.00 MPa (4,500 psig) with water or with a mixture of water and hydrogen or a mixture of water and syn gas for about 2-50 hours, whereby the conversion of said cobalt salts to said active cobalt catalyst species is improved when said salts are contacted with the treated preformer catalyst.