CO2-expanded liquids (CXLs) are demonstrated to be effective reaction media for the catalytic hydroformylation of I-octene. The performance of several rhodium catalysts, Rh(acac)(CO)(2), Rh(acac)[P(OPh)(3)](2), Rh(acac)(CO)[P(OAr)(3)], and two phosphorous ligands, PPh3 and biphephos, was compared in neat organic solvents and in CXLs wherein more than 50% of the solvent volume is replaced with dense CO2 at relatively mild temperatures (30-90 degrees C) and pressures ( < 12 MPa). For all catalysts, enhanced turnover frequencies (TOFs) were observed in CXLs. For the most active catalyst, Rh(acac)(CO)(2) modified by biphephos ligand, the selectivity to aldehyde products was improved from approximately 70% in neat solvent to nearly 95% in CXL media. The enhanced rates and selectivity are attributed to increased syngas availability in the CXL phase. In experiments performed without added solvent, a TOF maximum was observed at an optimum CO2 content It appears that at higher than optimum CO2 content, the TOF decreases as a result of dilution of substrate by CO2, whereas at lower than optimum values the TOF is limited by reduced syngas availability. The observed TOF (similar to 300 h(-1)), n/i ratio ( > 10), and aldehyde selectivity (similar to 90%) at the optimum CO2 content were either comparable to or better than values reported with other media and catalysts. Furthermore, the operating pressure (3.8 MPa) and temperature (60 degrees C) for the CXL process are significantly milder than those reported for industrial hydroformylation processes. (c) 2006 American Institute of Chemical Engineers.