The unimolecular photodissociation dynamics of acetone spanning the entire S₁ ← S₀ absorption spectrum have been reinvestigated, with a focus on mechanisms that produce CO. At excitation wavelengths of λ > 305.8 nm, all photoproducts are formed on the S₀ state after internal conversion. A roaming mechanism forming C₂H₆ + CO is active in the window λ = 311.2–305.8 nm. From λ = 305.8 to 262 nm, little or no CO is produced with the photochemistry dominated by the Norrish-type I C–C bond cleavage on the lowest excited triplet state, T₁. At higher energy ( λ < 262 nm), an increasing fraction of CH₃CO radicals from the primary reaction have sufficient internal energy to spontaneously decompose to CH₃ + CO. A new model is presented to account for the kinetic energy distribution of the secondary CH₃ radical, allowing us to determine the height of the energetic barrier to CH₃CO decomposition as 68 ± 4 kJ mol⁻¹, which lies midway between previous measurements. The fraction of CO from triple fragmentation rises smoothly from 260 to 248 nm. We see no evidence of the return of roaming, or any other S₀ reaction, in this higher energy region of the first electronic absorption band.