Provided is a process for producing alcohols or aldehydes by reacting monoolefins with carbon monoxide and hydrogen with less formation of by-products. The process comprises the step of reacting a monoolefin with carbon monoxide and hydrogen in the presence of a cobalt carbonyl catalyst until the conversion of monoolefin reaches 50-90% (the first reaction step), the step of separating unreacted monoolefin from the reaction mixture obtained in the first reaction step (the step of separation of unreacted monoolefin) and the step of reacting the separated unreacted monoolefin with carbon monoxide and hydrogen in the presence of a cobalt carbonyl catalyst (the second reaction step), wherein at least one of the first reaction step and the second reaction step is carried out in the presence of water.