Rhodium catalysts modified with triphenylphosphine, triphenyl phosphite, and tris(2,4-ditrrt-butylphenyl) phosphite have been evaluated for their performance in the hydroformylation of 1octene using carbon dioxide as the solvent. It is demonstrated that these catalysts are very efficient for the hydroformylation in carbon dioxide, although they are not designed for use in this medium. In particular, the catalyst prepared in situ from dicarbonyl(2,4-pentanedione)rhodium(I) and tris(2,4-di-tert-butyl-phenyl) phosphite gave rise to an initial turnover frequency in excess of 3x 10 4 molaldehyde molRh h-1. Such a reaction rate is unprecedented for hydroformylation in supercritical carbon dioxide-rich reaction mixtures. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA,.