Under CO pressure, the rhodium/ionic-iodide system catalyzes either the reductive carbonylation of methyl formate into acetaldehyde or its homologation into methyl acetate. The influence of the reaction conditions on the selectivity of these two react-ions was investigated and it was found that the former occurs selectively only in N-methylpyrrolidone (NMP) (or related solvents), for low I-/Rh ratio, low substrate concentration, and high CO pressure, whereas methyl acetate is preferentially formed, in the same solvent, at higher I- and substrate concentrations and under lower CO pressure. By using labeled methyl formate ((HCO2CH3)-C-13) it was also shown that the carbonyl group of acetaldehyde or methyl acetate does not result from that of methyl formate. In situ IR studies conducted under catalytic conditions (high-pressure, high-temperature) have not enabled the identification of any other catalytic species than RhI2(CO)(2)(-) (which is also the active species in methanol or methyl acetate carbonylations), whatever the reaction conditions ([I-], P-CO...). Plausible mechanisms are proposed for these reactions in which the essential role played by NMP in controlling the CH3I content in the reaction medium is clarified and taking into account these experimental data.