The solubility of carbon monoxide in 37 ionic liquids and in some organic solvents has been determined using high-pressure C-13 NMR spectroscopy; a method for predicting the CO solubility is demonstrated, and it was shown that the rate of the hydroformylation of 5-hexen-2-one does not correlate with the CO solubility, as expected from the determined relative solubility of CO compared to H-2.