Vapour phase hydroformylation of ethylene was studied with alumina supported Co, Ni, Mo, CoMo and NiMo sulfide catalysts in a flow reactor with fixed bed at 513-563 K, 10 - 10(5) Pa of overall pressure in the presence of H2S in the feed. With this reaction mixture three different kinds of reactions were observed, hydrogenation, hydroformylation and H2S addition. Nickel and cobalt catalysts presented the best hydroformylation activity. The addition of molybdenum enhanced the formation of ethane and sulfur compounds but decreased the hydroformylation reaction. This last result is discussed in relation with the formation of a NiMoS or CoMoS phase.