The quantum yield of CO in the laser pulse photolysis of acetone at 248 nm and at 298 K in the pressure range 20–900 mbar (N₂) has been measured directly using quantitative infrared diode laser absorption of CO. It is found that the quantum yield of CO shows a significant dependence on total pressure with Φ_CO decreasing with pressure from around 0.45 at 20 mbar to approximately 0.25 at 900 mbar. From a combination of ab initio quantum chemical calculations on the molecular properties of the acetyl (CH₃CO) radical and its unimolecular fragmentation as well as the application of statistical (RRKM) and dynamical calculations we show that CO production results from prompt secondary fragmentation (via (2a)) of the internally excited primary CH₃CO* photolysis product with an excess energy of approximately 62.8 kJ mol⁻¹. Hence, our findings are consistent with a consecutive photochemically induced decomposition model, viz. step (1): CH₃COCH₃ + hv → CH₃CO* + CH₃, step (2a): CH₃CO* → CH₃ + CO or step (2b) CH₃CO* –(+M)→ CH₃CO. Formation of CO via a direct and/or concerted channel CH₃COCH₃ + hv → 2CH₃ + CO (1′) is considered to be unimportant.