The kinetics of the CH2CHO + O-2 reaction was experimentally studied in two quasi-static reactors and a discharge flow-reactor at temperatures ranging from 298 to 660 K and pressures between 1 mbar and 46 bar with helium as the bath gas. The CH2CHO radicals were produced by the laser-flash photolysis of ethyl vinyl ether at 193 nm and by the reaction F + CH3CHO, respectively. Laser-induced fluorescence excited at 337 or 347.4 nm was used to monitor the CH2CHO concentration. The reaction proceeded via reversible complex formation with subsequent isomerization and fast decomposition: CH2CHO + O-2 <-> O2CH2CHO -> HO2CH2CO -> products. The rate coefficients for the first and second steps were determined (k(1), k(-1), k(2)) and analyzed by a master equation with specific rate coefficients from the Rice-Ramsperger-Kassel-Marcus (RRKM) theory. Molecular and transition-state parameters were obtained from quantum chemical calculations. A third-law analysis led to the following thermodynamic parameters for the first step: Delta S-R degrees(300K(l)) = -144 J K-1 mol(-1) (1 bar) and Delta H-R degrees(300K(l)) = (-101 +/- 4) kJ mol(-1). From the falloff analysis, the following temperature dependencies for the low- and high-pressure limiting rate coefficients were obtained: k(1(0)) = 5.14 x 10(-14) exp(210 K/T) cm(-3) s(-1); k(1(infinity)) = 1.7 x 10(-12) exp(-520 K/T) cm(-3) s(-1); and k(2(infinity)) = 1.3 x 10(12) exp[-(82 +/- 4) kJ mol(-1)/RT] s(-1). Readily applicable analytical representations for the pressure and temperature dependence of k(1) were derived to be used in kinetic modeling.