Abstract: Process to separate a rhodium/phosphite ligand complex and free phosphite ligand from a hydroformylation mixture also containing high boiling hydroformylation compounds by contacting said mixture with a polymeric membrane, wherein said mixture is diluted with at least one diluent compound having a solubility parameter such that the absolute difference in relation to the solubiltity parameter of the polymeric membrane lies between 0 and 400{square root}{square root over (KJ/m3)} and the ratio of the molar volume of the phosphite ligand and said diluent compound is ≧1.5, to such an extent that the amount of high boiling compounds in said mixture is ≦50 wt. % (relative to the total amount of the mixture) and the amount of compounds with a molecular weight of between x ±200 is ≦20 wt. % (relative to the total amount of the mixture) whereby x is the cut-off of the membrane (g/mole).