In a hydroformylation reaction, the circulation and recovery of cobalt carbonyl in a cobalt catalyst cycle using carbonylate as an intermediate and acidification thereof, is improved by supplying an aqueous sulfuric acid solution containing less than 16% wt of sulfuric acid to the carbonylate to control sulfuric acid concentration in the carbonylate/acid mixture, before accounting for any acid consuming reaction, to a value of from 2% to 10% by weight. Single step and multistage liquid/liquid extraction techniques as well as vapor/liquid extraction techniques followed by the absorption of cobalt from the vapor in an organic liquid both benefit from this improvement.