The reaction of acetaldehyde with atomic oxygen, O(3P), was studied by the discharge-flow-photoionization mass spectrometry method at room temperature (295 ± 4 K) for the conditions of excess atomic oxygen. The rate constant for reaction 1, CH3CHO + O → CH3CO + OH, was (3.9 ± 0.3) × 10-13 cm3 molecule-1 s-1. Branching fractions for the subsequent reactions 2a, CH3CO + O → CH2CO + OH, and 2b, CH3CO + O → CH3 + CO2, were determined to be 22 ± 5% and 76 ± 24%, respectively. The experiments in the presence of molecular oxygen show that the ratio of rate constants for reaction 3, CH3CO + O2 + M → CH3CO3 + M, to reaction 2 is (6.3 ± 0.5) × 10-3 [total pressure 3.6 Torr (He)]. From the rate constant for reaction 3 obtained by McDade et al., the rate constant for reaction 2 was calculated: k2 = (3.2 ± 0.7) × 10-10 cm3 molecule-1 s-1. When the concentration of molecular oxygen increased, the yields of both CH3 and CH2CO decreased. From this decrease, the branching fractions for reactions 4a, CH3CO3 + O → CH2CO + OH + O2, and 4b, CH3CO3 + O → CH3 + CO2 + O2, were determined: < 10% and 27 ± 25%, respectively. A discussion of the other products from CH3CO3 + O is presented.