We report measurements of the CH 3 CO quantum yield, ΦCH3CO, following the 248nm pulsed laser photolysis of acetone (CH 3 C(O)CH 3 ), methyl ethyl ketone (CH 3 C(O)CH 2 CH 3 ), and biacetyl (CH 3 C(O)C(O)CH 3 ). CH 3 CO quantum yields at 248nm were measured at 296K, relative to CH 3 CO reference systems. CH 3 CO was detected using cavity ring-down spectroscopy at wavelengths between 490 and 660nm. Measurements were performed between 60 and 670Torr (He, N 2 bath gases) and the obtained CH 3 CO quantum yields in the low-pressure limit, ΦCH3CO0, were 0.535±0.09, 0.41±0.08, and 0.76±0.11, for acetone, methyl ethyl ketone, and biacetyl, respectively. The quoted uncertainties are 2σ (95% confidence level) and include estimated systematic errors. An increase in ΦCH3CO with increasing bath gas pressure, which depended on the identity of the collision partner (He, N 2 ), was observed. The present results are compared with previous quantum yield determinations.