The reaction of 1-methylvinoxy radicals, CH3COCH2, with molecular oxygen has been investigated by experimental and theoretical methods as a function of temperature (291-520 K) and pressure (0.042-10 bar He). Experiments have been performed by laser photolysis coupled to a detection of 1-methylvinoxy radicals by laser-induced fluorescence LIF. The potential energy surface calculations were performed using ab inito molecular orbital theory at the G3MP2B3 and CBSQB3 level of theory based on the density function theory optimized geometries. Derived molecular properties of the characteristic points of the potential energy surface were used to describe the mechanism and kinetics of the reaction under investigation. At 295 K, no pressure dependence of the rate constant for the association reaction has been observed: k(1,298)K) (1.18 +/- 0.04) x 10(-12) cm(3) s(-1). Biexponential decays have been observed in the temperature range 459-520 K and have been interpreted as an equilibrium reaction. The temperature-dependent equilibrium constants have been extracted from these decays and a standard reaction enthalpy of Delta H-r,H-298K = 105.0 +/- 2.0 kJ mol(-1) and entropy of Delta Sr,298K) - 143.0 +/- 4.0 J mol(-1) K-1 were derived, in excellent agreement with the theoretical results. Consistent heats of formation for the vinoxy and the 1-methylvinoxy radical as well as their O-2 adducts are recommended based on our complementary experimental and theoretical study Delta H-f,H-298K = 13.0 +/- 2.0, -32. 9 +/- 2.0, -85.9 +/- 4.0, and - 142.1 +/- 4.0 kJ mol(-1) for CH2CHO, CH3COCH2 radicals, and their adducts, respectively.