CO2-expanded liquids (CXLs) represent a continuum of reaction media that combine the reaction benefits provided by organic solvents and the environmental benefits provided by compressed CO2 (liquid and scCO(2)) in an optimal manner. Homogeneous hydroformylation of I-octene using an unmodified rhodium catalyst (Rh(acac)(CO)2) was successfully demonstrated employing CXLs as reaction media. At temperatures of 30 and 60degreesC, the turnover numbers (TONs) for aldehyde formation in CO2-expanded acetone were up to fourfold higher than those obtained in either neat acetone or compressed CO2. The higher TON in CXLs is attributed to enhanced syngas solubility in CXL (compared to neat solvent) while maintaining complete miscibility of the transition metal catalyst complex (due to the presence of the neat solvent). The demonstrated advantages of using CXLs include significant replacement of conventional solvents (up to 80 % by volume) by environmentally benign CO2 and process intensification at relatively mild pressures (compared to those required with scCO(2))The regioselectivity towards linear and branched aldehydes (n/iso ratio) remained unaffected by the change in either the solvent media or the temperature. The observed effects of temperature and H-2 concentration in CXL media were similar to those reported in the literature: lower temperatures favored the selectivity toward the aldehydes while an increase in H2 concentration resulted in a higher reaction rate. These first results point to the need for fundamental investigations of the catalyst and reactant solubilities and of the underlying reaction mechanism in CXLs to better understand and exploit these promising reaction media in catalytic hydroformylations. (C) 2004 Elsevier Ltd. All rights reserved.