The selective hydrogenation of cinnamaldehyde and other unsaturated aliphatic aldehydes under a two-phase liquid system using toluene and aqueous alkaline solution over supported palladium catalysts has been investigated with a view to obtaining the corresponding saturated aldehydes with sufficiently high selectivities. In the case of cinnamaldehyde. the effects of various process parameters on the reaction rate and selectivity with respect to the aldehyde have been investigated. The results obtained are likely to be useful for the rational design of the process for the production of industrially relevant products, such as hydrocinnamaldehyde and citronellal using cinnamaldehyde and citral as the starting materials.