The addition of cetyltrimethylammonium bromide (CTAB) to TPPTS/rhodium hydroformylation catalysts has a complicated effect on reaction activity and selectivity. In water alone as the solvent, high CTAB concentration leads to the formation of emulsions and reaction selectivity drops. In aqueous alcohol solvents selectivity also drops but the effect appears to be due to the solvent composition. Emulsion formation is minimized and initial reaction activity goes through a maximum at a CTAB/TPPTS ratio of 3. NMR studies show that there is a strong interaction of CTAB and TPPTS in water while the interaction in aqueous methanol appears to be nonspecific. (C) 2002 Elsevier Science B.V. All rights reserved.