The kinetics of 1-octene hydroformylation catalyzed by triphenylphosphine modified rhodium in carbon dioxide have been explored at 90 °C and pressures up to 48 MPa. The apparent catalyst solubility was determined by evaluating the reaction rate for different rhodium amounts. The kinetics follow a first order in 1-octene, a negative order of -1.2 in carbon monoxide, an order of 0.25 in hydrogen, and a negative order of -0.2 in triphenylphosphine, which is to a great extent in agreement with studies on the hydroformylation of linear 1-alkenes in organic solvents. The observed apparent turnover frequencies range from 1900 to 7000 molaldehyde molRh-1 h-1. These turnover frequencies are of the same order of magnitude as observed for hydroformylations in organic solvents, indicating that rhodium modified with triphenylphosphine can be used with high efficiency in supercritical carbon dioxide rich mixtures.