Process to separate rhodium/phosphite ligand complex and free phosphite ligand complex from hydroformylation mixture

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 membrane lies between 0 and 400kJ/m 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.% and/or the amount of compounds with a molecular weight of between x +/- 200 is ≤ 20 wt.% whereby x is the cut-off of the membrane (g/mole) (relative to the total amount of the mixture).