We report measurements of the CH3 CO quantum yield, ˚CH3 CO , following the 248 nm pulsed laser photolysis of acetone (CH3 C(O)CH3 ), methyl ethyl ketone (CH3 C(O)CH2 CH3 ), and biacetyl (CH3 C(O)C(O)CH3 ). CH3 CO quantum yields at 248 nm were measured at 296 K, relative to CH3 CO reference systems. CH3 CO was detected using cavity ring-down spectroscopy at wavelengths between 490 and 660 nm. Measurements were performed between 60 and 670 Torr (He, N2 bath gases) and the obtained CH3 CO quantum yields in the low-pressure limit, ˚0CH CO , were 0.535 ± 0.09, 0.41 ± 0.08, and 0.76 ± 0.11, for acetone, methyl

3 ethyl ketone, and biacetyl, respectively. The quoted uncertainties are 2 (95% confidence level) and include estimated systematic errors. An increase in ˚CH3 CO with increasing bath gas pressure, which depended on the identity of the collision partner (He, N2 ), was observed. The present results are compared with previous quantum yield determinations.