For ultrafiltration separation of a rhodium hydroformylation catalyst with a polymer ligand that is dissolved in the reaction mixture, an investigation was made of the applicability of acetylcellulose, polysulphoneamide, and polyamide membranes, and also membranes manufactured from a copolymer of vinylidenefluoride and tetrafluoroethylene. Effective retention of rhodium complexes with phosphinated polystyrene (molecular weight (MW) 270,000) and polyvinyl chloride (MW 170,000) was achieved on partially hydrolysed membranes of cellulose acetate, which, before hydrolysis, had pore diameters of 100-300 Angstrom, and on membranes of polysulphonamide with pores of 50 and 100 Angstrom. Rhodium complexes with phosphinated polybutadiene (MW 190,000) can be separated by using polysulphonamide membranes with pores of 50 Angstrom. Polysulphonamide membranes retain their productivity and retention capacity after prolonged (7200 hr) contact with the reaction mixture. The catalytic properties of rhodium complexes with phosphinated polystyrene remain constant over the course of six hydroformylation-ultrafiltration cycles.