Acetonyl radicals and acetonylperoxy radicals were produced by pulse radiolysis of gas mixtures with varying concentrations of CH₃COCH₃ and O₂ in 1 atm of SF₆ to initiate the reactions (1) F+CH₃COCH₃→HF+CH₃COCH₂, (2) 2CH₃COCH₂ → products, (3) CH₃COCH₂+O₂(+M) → CH₃COCH₂O₂(+M) and (4) 2CH₃COCH₂O₂ → products. The ultraviolet absorption spectrum of the acetonyl radical is composed of a fairly weak band centered at 315 nm and a stronger band in the range of 200-250 nm. The absorption cross section has been determined, σ(310 nm) = (8.73 ± 0.50) × 10^-19 cm² molecule^-1. The values k₂=(4.8 ± 0.4) × 10^-11, k₃=(1.5 ± 0.3) × 10^-12 and k₄=(8.3 ± 1.6) × 10^-12 cm³ molecule^-1 s^-1 at 298 K were derived by computer modelling of the observed kinetic features.