The OH-initiated oxidation of acetone in aqueous solution is investigated because of its potential implications in atmospheric chemistry. The UV-spectrum of the transient acetonylperoxy radical was measured. Two characteristic absorption bands of the acetonylperoxy radical spectrum are found in the 220-400 nm wavelength region. The rate constant for the recombination reaction of the acetonylperoxy radical was determined as a function of temperature for the first time in aqueous solution with K-rec,K-298 K = (7.3 +/- 1.3) x 10(8) M-1 s(-1), E-A = 4.5 +/- 3.3 kJ mol(-1), and A = (4.7 +/- 2.7) x 10(9) M-1 s(-1). Furthermore, kinetic investigations of the OH-initiated oxidation of methylglyoxal and pyruvic acid were performed with the following results: for methylglyoxal, k(second) = (6.2 +/- 0.2) x 10(8) M-1 s(-1), E-A = 12 +/- 2 kJ mol(-1), and A = (7.8 +/- 0.2) x 10(9) M-1 s(-1); for pyruvic acid (pH = 0), k(second) = (3.2 +/- 0.6) x 10(8) M-1 s(-1), E-A = 15 +/- 5 kJ mol(-1), and A = (1.1 +/- 0.1) x 10(11) M-1 s(-1); for pyruvate (pH = 6), k(second) = (7.1 +/- 2.4) x 10(8) M-1 s(-1), E-A = 25 +/- 19 kJ mol(-1), and A = (1.5 +/- 0.4) x 10(13) M-1 s(-1). Quantitative product studies were done as a function of the number of laser photolysis pulses for acetone and its oxidation products methylglyoxal, hydroxyacetone, pyruvic acid, acetic acid, and oxalic acid. After the recombination reaction of acetonylperoxy radicals, there are two possible decomposition reactions where the primary products methylglyoxal and hydroxyacetone are formed. From product analysis after a single photolysis laser shot, the ratio of the main product-forming reactions was determined as (A) 30% and (B) 56% for the methylglyoxal formation via channel A to yield two molecules of methylglyoxal and channel B to yield one molecule of methylglyoxal and one molecule of hydroxyacetone. The remaining product can be ascribed to channel C, the radical-retaining channel forming alkoxy radicals with a yield of 14%. Pyruvic acid and acetic acid were found to be the major intermediates estimated with concentrations in the same order of magnitude and a similar time profile, indicating that acetic acid is also a possible oxidation product of methylglyoxal.