The effect of the presence of chlorine on the activity and selectivity of a Cu/Al2O3 catalyst has been examined for the selective hydrogenation of an alphabeta unsaturated aldehyde, crotonaldehyde. Cu/Al2O3 in the absence of chlorine poisons produced 1-butanol almost exclusively, whereas catalysts pre-dosed with a suitable amount of chlorine compound (CCl4, CHCl3 and CH2Cl2) shifted the product distribution towards formation of butanal. The poisoning effectiveness increased in the order CCl4 < CHCl3 < CH2Cl2 much less than CH3Cl and methyl chloride was found to totally deactivate the catalyst. The most significant enhancement in butanal selectivity was observed with CCl4 and CH2Cl2. The effect of chlorine as a poison is in contrast to the effect of sulphur which enhances formation of crotyl alcohol and the origins of these effects are discussed.