The catalytic hydrocarboxylation of linear alkenes to carboxylic acids using supercritical carbon dioxide as a solvent was studied. High selectivities in acids have been obtained. The best results were achieved when adding a perfluorinated surfactant to the reaction mixture (93% conversions and ca. 80% selectivity). Comparative multinuclear high-pressure NMR spectroscopic studies in [D8]THF and in supercritical CO2 show the formation of Pd0 species.