Abstract: In converting an alkene such as ethylene to a carbonyl derivative such as acetaldehyde by a process which comprises passing the alkene through a tubular reactor along with an aqueous acidic catalyst solution comprising a noble metal such as palladium together with a cupric chloride redox agent, followed by separating the carbonyl product from the depleted catalyst solution which is then reoxidized with a source of molecular oxygen such as air prior to being returned to the reactor, continuous depletion of the chloride content of the catalyst solution takes place as a result of the formation of chlorinated reaction by-products. This necessitates replenishment of the chloride content of the catalyst solution by adding hydrochloric acid into a catalyst regeneration zone preceding the catalyst reoxidation step of the process.