Carbonyl compounds are produced by the oxidation of an olefine with oxygen in a reaction zone at a temperature in the range 50-250 DEG C. under a pressure from atmospheric to about 50 atm. and in the presence of a liquid catalyst comprising a noble metal of Group VIII of the Periodic Table and a redox system, the reaction products being cooled so that a portion of the carbonyl compound is condensed and this portion being returned to the reaction zone. If desired a second portion of the carbonyl, which may be the remainder of it, may be also condensed and subsequently withdrawn. Alternatively a third portion of the carbonyl compound may be recycled to the reaction zone as a vapour. The oxygen used for the oxidation may be pure oxygen, a gaseous mixture of oxygen with an inert gas, e.g. nitrogen or carbon dioxide, or may be produced from a compound yielding oxygen under the reaction conditions. The portion of the carbonyl compound condensate returned to the reaction zone may be introduced as a reflux into a fractionating column communicating with the reaction zone. The carbonyl compound condensate may be fractionated in a separate stage to separate higher boiling by-products before it is introduced into the fractionating column. Examples are given of the conversion of ethylene to acetaldehyde. Specifications 898,790, 900,829, 922,694, 938,831, 938,832, 938,833, 938,834, 938,835 and 938,836 are referred to.