Acetaldehyde is prepared in good yield from methanol and synthesis gas under mild reaction conditions by contacting a mixture of methanol, carbon monoxide and hydrogen with an iodide or iodine free catalyst composition comprising (1) ruthenium powder, (2) a cobalt-containing compound (3) a rhodium-containing compound, and (4) an onium salt or base, and heating the resulting mixture under mild temperature and pressure for sufficient time to produce the desired acetaldehyde, and then recovering the same from the reaction mixture.