The new paradigm of pollution prevention dictates that we develop new techniques for producing valuable commodity chemicals that are sustainable and present minimal wastes. One appropriate technique is solvent replacement. Within this paper, we investigate the use of supercritical carbon dioxide as a replacement solvent for hydroformylation, used commercially in the oxo process to produce over 1 x 10(9) kg of aldehyde annually. Unfortunately, commercial hydroformylation catalysts are not soluble in supercritical CO2, thus we describe the development of a heterogeneous catalyst for use with this reaction. Rhodium, supported on activated carbon, is active for the hydroformylation of propene to butanal, but it is not selective. Two different activated carbons are used to provide information as to the role of the support in promoting this reaction. A two-step reaction pathway is defined, and the effect of temperature and pressure are quantified based on the proposed pathway. (C) 2000 Elsevier Science B.V. All rights reserved.