Acyl radicals RCO are important intermediates formed after reaction of aldehydes with OH or NO3 radicals. Like most alkoxyl radicals, under atmospheric conditions, they can react via 3 different reaction pathways : unimolecular thermal decomposition, possibly in the fall-off range, isomerization, and reaction with O2. Since isomerization is negligible for small radicals, the ozone creating potential of acyl radicals is only determined by the ratio between decomposition and reaction with O2. Using our newly developed Pulsed Laser Photolysis / Tunable Diode Laser spectrometer (Hanoune et al. 2001), we have measured directly the decomposition rate constant of two acyl radicals (2,2-dimethylpropionyl or pivaloyl (CH3)3CCO, and isobutyryl (CH3)2CHCO), between 300 and 318 K and between 4.7 and 34 Torr. Reactions were monitored by probing the build-up of CO. It was found to be necessary to work in presence of O2 to avoid Cl regeneration. Our data complement the few existing measurements performed at higher pressure. Jagiella et al. (2000) measured the ratio between decomposition and O2 reaction in a photochemical reaction chamber combined with long-path FTIR analysis between 293 and 317 K at atmospheric pressure. Concurrently, Tomas et al. (2000) measured the same ratio by flash photolysis coupled to UV absorption spectrometry at atmospheric pressure between 323 and 453 K, with a few additional measurements in the fall-off domain (433 K, 40 and 150 Torr).