Aliphatic and cycloaliphatic aldehydes are obtained from olefins and cycloalkenes (boiling not below the reaction temperature), H2 and CO at a temperature above 80° and at a pressure of 10-50 atm. in the presence of a Co-containing catalyst. A preferred catalyst is partly reduced CoO activated with up to 2% Fe2O3 and supported on a carrier. Thus, to Me (CH2) 15CH:CH2 1 and catalyst 0.1 kg. (obtained by partial reduction of basic carbonates of Co 98% and Fe 2% with H2 at 350°) in a 3-1. tank are added CO and H2 at a pressure of 12 atm. The mixture is heated at 100°-110° and the addition of CO and H2 continued until absorption ceases, to yield mainly Me (CH2) 16CHMeCHO and Me(CH2) 18CHO containing small quantities of the corresponding alcohols. Gives other examples.