The continuous flow hydroformylation of 1-octene catalysed by Rh/[RMIM][Ph2PC6H4SO3] (R = 1-propyl, 1-pentyl or 1-octyl) dissolved only in the steady state reaction mixture and using scCO2 as a transport vector for both substrates and products gives rates up to 160–240 catalyst turnovers h−1 with low rhodium leaching over a 12 h period at a total pressure of 125–140 bar.