The influence of the cationic promoters (M = Li, Na, K, Rb, Cs) on silica-supported rhodium catalysts was examined in the hydroformylation of propene at 413 K and atmospheric pressure. The catalysts were prepared by aqueous co-impregnation of RhCl3.3H(2)O and alkali-metal chloride. The rhodium to promoter molar ratio was 1. A pre-treatment cycle under O-2/H-2/O-2 at 673 K was carried out on the catalyst before the admission of the hydroformylation mixture. By in situ Rh K-edge extended X-ray adsorption fine-structure (EXAFS) characterization of the activated catalyst, two different Rh and alkali-metal oxide phases, highly dispersed and in intimate contact with each other, were detected on the support surface. Under catalytic conditions, small Rh metal particles are formed, with a mean diameter of 3.2 nm, as detected by HRTEM. The activity in aldehyde formation depends on the alkali-metal promoter in the following order: Li > Na > K > Rb much greater than Cs. This behavior parallels the trend of the polarizing power of the cation within the Group 1. (C) 2003 Elsevier Science B.V. All rights reserved.