For the hydroformylation of 1-octene in supercritical carbon dioxide the relationship between the change in pressure and the change in reaction mixture composition as a function of time has been investigated. The activity and selectivity has been studied for the catalyst based on tris(3,5-bis(trifluoromethyl)phenyl)phosphine and rhodium(I) dicarbonyl acetylacetonate. The influence of the ligand to rhodium ratio on the hydroformylation has been used to demonstrate how the pressure can be correlated to the conversion and yield. The initial rate of reaction is in good agreement with the initial pressure change in the batch reactor. Up to an aldehyde yield of 80%, the pressure drop appears to be independent of the reaction rate and selectivity. The highest average reaction rate, , has been obtained for a ligand to rhodium ratio of 50 and an initial concentration of 1-octene of 0.5 mol L-1. Both the reaction rate and the selectivity increase when the ligand to rhodium ratio is increased. The Peng–Robinson equation of state has been used to describe the pressure as a function of the concentration of the reactants and products. The calculated pressure corresponds reasonably well with the observed reactor pressure. Following the progress of the reaction by monitoring the pressure is a good alternative to reaction mixture sampling, especially for fast reactions.