We report measurements of the CH3CO quantum yield, ̊CH3CO, following the 248nm pulsed laser pho-tolysis of acetone (CH3C(O)CH3), methyl ethyl ketone (CH3C(O)CH2CH3), and biacetyl (CH3C(O)C(O)CH3).CH3CO quantum yields at 248nm were measured at 296K, relative to CH3CO reference systems. CH3COwas detected using cavity ring-down spectroscopy at wavelengths between 490 and 660nm. Measure-ments were performed between 60 and 670Torr (He, N2bath gases) and the obtained CH3CO quantumyields in the low-pressure limit, ̊0CH3CO, were 0.535±0.09, 0.41±0.08, and 0.76±0.11, for acetone, methylethylketone,andbiacetyl,respectively.Thequoteduncertaintiesare2(95%confidencelevel)andincludeestimated systematic errors. An increase in ̊CH3COwith increasing bath gas pressure, which dependedon the identity of the collision partner (He, N2), was observed. The present results are compared withprevious quantum yield determinations.