Process for separating a dissolved complex catalyst of a metal of group 4, 5, 6, 7, 8, 9 or 10 of the Periodic Table of the Elements and/or any free organophosphorus ligand present from a nonaqueous hydroformylation reaction mixture which contains an aldehyde product and an organic solvent at least one membrane which is more permeable to the hydroformylation product than to the organophosphorus ligand, the separation being carried out under a carbon monoxide partial vapor pressure of more than 200 kPa.