Abstract: In a hydroformylation process for the preparation of aldehydes by reacting aliphatic olefins of 2 to 12 carbon atoms with carbon monoxide and hydrogen at temperatures of 20.degree. to 150.degree. C. and pressures of 0.1 to 20 mPa in the liquid phase and in the presence of an aqueous solution containing 50 to 800 wt. ppm of rhodium and 25 to 30 wt. % of phosphines, in each case based on the aqueous solution as a catalyst system and in addition the transformation products of the phosphines, the improvement comprising draining off a portion of the aqueous solution of the catalyst system together with the reaction product and replacing the same by fresh aqueous solution of the catalyst system whereby the original catalyst selectivity for formation of n-aldehydes is maintained.