\ Methyl acrylates were hydroformylated with high regioselectivity and excellent yield to the corresponding branched aldehydes using Rh/PPh3-supported ionic liquid-phase (SILP) catalyst in water as a reaction medium. The effect of various reaction variables like temperature, pressure, P/Rh ratio, substrate/Rh ratio, and phosphorous ligands on chemoselectivity and regioselectivity for hydroformylation reaction were optimized. The optimized reaction conditions were then applied for hydroformylation of various unsaturated esters. The methyl acrylate hydroformylation products were further reacted with phenyl hydrazine to obtain, 4-methyl-substituted pyrazolin-5-ones with high yield. (C) 2008 Elsevier B.V. All rights reserved.