Flash photolysis UV absorption techniques were used to study the HO2 + CH3C(O)O-2 reaction. It was found that the reaction HO2 + CH3CHO reversible arrow CH3CH(OH)O-2 (2, -2) can interfere with the kinetic measurements. Thus, the kinetics and thermochemistry of this reaction were investigated. The UV spectrum of the CH3CH-(OH)O-2 radical was determined, and the rate constant for the association reaction was measured to be k(2) = 4.4 x 10(-14) cm(3) molecule(-1) s(-1) at 298 K, with an uncertainty factor of about 2. The reaction was found to be equilibrated near room temperature, and the equilibrium constant was determined between 298 and 373 K: K-2 = k(2)/k(-2) = 1 9 X 10(-(27 +/-1)) exp[+(6925 +/- 840 K)/T] cm(3) molecule(-1). This corresponds to DeltaH degrees (298) = -64 +/- 8 kJ mol(-1) and DeltaS degrees (298) = -157 +/- 6 J K-1 mol(-1) terrors are 2 sigma, the latter value being in good agreement with the calculated value, using the AM1 semiempirical method. The kinetics of the reactions CH3C(O)O-2 + HO2 --> CH3C(O)OOH + O-2 (1a) and CH3C(O)O-2 + HO2 --> CH3C(O)OH + O-3 (1b) were determined in the temperature range 273-403 K, using low acetaldehyde concentrations, so that reaction 2 could be neglected. The initial radical concentrations were measured carefully and were consistent with the concentrations of the precursors (acetaldehyde and methanol). The rate expression is k(1) = (6.4 +/- 2.5) x 10(-13) exp[+(925 +/- 120 K)/T] cm(3) molecule(-1) s(-1), yielding k(1) = (1.42 +/- 0.07) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K. The quoted errors are 2 sigma, and the global uncertainty is estimated at 158. The branching ratio of channel 1b, beta (1) = k(1b)/k(1), was determined by measuring the residual absorption, from 230 to 280 nm, at sufficiently long time, so that all radicals had recombined. It was verified that the UV spectrum of the residual absorption was the same as that of ozone, within uncertainty. The resulting value beta (1) = 0.20 +/- 0.02 was derived, independent of temperature over the range studied.