The kinetics of biphasic hydroformylation of 1-octene has been studied in the presence of ethanol cosolvent, using water-soluble Rh/TPPTS as a catalyst. The rate was found to be first order with catalyst and olefin concentrations and fractional order with hydrogen concentration. The rate versus CO concentration passed through a maximum, indicating a negative order dependence at higher pressures. These trends have been interpreted on the basis of a well-known hydroformylation mechanism. A rate equation derived assuming the addition of olefin to the active catalyst as a rate-determining step was found to fit the data satisfactorily. The kinetic parameters were evaluated for different temperatures.