Iridium complexes Ir-4(CO)(12), IrCl3, and [IrCl(CO)(3)]n show activity toward hydroformylation of 1-hexene in the presence of inorganic salts as promoters. Use of promoters such as LiCl, Li2CO3, KCl, CaCl2, and LiBr enhanced the catalytic activity and aldehyde selectivity considerably by suppressing the competing hydrogenation processes. The activity of the iridium compounds increases in the order IrCl3 < [IrCl(CO)(3)]n < Ir-4(CO)(2). The role of the promoter and the effect of the reaction conditions are discussed. (C) 2003 Elsevier Science (USA). All rights reserved.