Rhodium complexes show high activity in homogenous carbonylations of alcohols and hydrocarboxylations of alkenes to carboxylic acids in the presence of alkyl halides. Analogously to liquid phase reactions, rhodium on different supports (active carbon, zeolite, SiO2, A1203, polymers, etc.) is the most active and selective among transition metal catalysts for vapor phase carbonylations; very good results were obtained with rhodium on active carbon. In this paper Fourier transform infrared (FT-IR) transmission spectroscopy was used to verify the validity of homogeneous methanol carbonylation mechanisms for heterogeneous phase applications. FF-IR spectra of adsorbed species obtained on RhCl3/Cab-O-Sil 5 catalyst (rhodium content 2.85%) pressed into a self-supporting pellet and treated successively with CO, CH3I and CH3OH, gave valuable information on the reaction mechanism. The catalytically active complexes ([Rh(CO)2X2]-where X = C1, I or more correctly [Rh(CO)2XxLy ]-where L = OH-or H20 and x +y = 2) were formed from supported RhC13 • 3H20 , CO and CH3I under relatively mild conditions: Pco = 1 bar, PMel = 5 mbar, T= 305-353 K. The absorption band appearing at 1717-1723 cm-1 after CH3I adsorption on the catalyst surface is close to the acetyl ~O stretching frequency (1708-1711 cm -1) obtained under homogeneous conditions, which proves the similarity of reaction mechanisms in both liquid and vapor phases.