A three-phase catalytic membrane reactor (3PCMR), designed for the evaluation of catalytic membranes, was successfully applied to carry out selective oxidation of ethylene to acetaldehyde under mild conditions. The reaction was performed over carbon supported Pd and Pt partial hydrophobic catalytic membranes by using different redox couples as reaction medium. Results have shown that reaction proceeds with relevant rate and high acetaldehyde selectivity (> 97%). As expected, the Pd membrane was found to be more active and selective than the Pt membrane. 3PCMR allows continuous separation of acetaldehyde while the reaction proceeds, therefore its employment could offer an original alternative to overcome several engineering problems of the conventional Wacker process. A reaction model adequate to describe the reaction system is proposed.