Rh(I) and Rh(III) impregnated on mesoporous supports such as MCM-41 have demonstrated a very high catalytic activity in the production of acetals under the hydroformylation conditions of alkenes. The one pot process that combines hydroformylation and acetalization processes leading to acetals as major products was carefully studied. R(I) supported catalysts combined with the heteropolyacid H3PW12O40 showed high catalytic activity towards the formation of acetals. However, Rh(III) supported catalysts were much more active in the absence of any additive. The addition of P(OPh)(3) increased significantly the selectivity of the reaction towards the branched acetals. The effects of the addition of different types of phosphine and phosphite ligands were studied. (c) 2006 Elsevier B.V. All rights reserved.