Two different in situ prepared catalysts generated from Rh(CO)2acac and trifluoromethyl-substituted triphenylphosphine ligands have been evaluated for their activity and selectivity in the hydroformylation of 1-octene. The solvents used were supercritical carbon dioxide, hexane, toluene, and perfluoromethylcyclohexane. The highest value for the turnover frequency, 9820 mol1-octene molRh-1 h-1, has been obtained in supercritical carbon dioxide using ligand I, P[C6H3-3,5-(CF3)2]3. For both supercritical carbon dioxide and hexane employing ligand II, P(C6H4-3-CF3)3, a selectivity towards the linear aldehyde product, nonanal, and an n:iso ratio of 79.3¿% and 4.6–4.8 have been obtained, respectively. These values are significantly higher than those obtained with triphenylphosphine as ligand (nonanal: 74–76¿%, n:iso: 3.1–3.3). An increase in trifluoromethyl substitution on the triphenyl ligand results in an increase in the 1-octene conversion rate, an increase in the n:iso ratio and a decrease in the overall selectivity towards aldehydes. In terms of turn-over frequency and selectivity the three ligands give comparable results in supercritical carbon dioxide and hexane. This leads to the conclusion that the properties of supercritical carbon dioxide as a solvent for hydroformylation can be better compared with those of hexane rather than with those of toluene.