A mechanistic and kinetic study of the OH.-initiated oxidation of benzaldehyde is carried out using quantum chemical methods and classical transition state theory. We calculate the rate constant for this reaction within the temperature range of 200–350 K at atmospheric pressure. All possible hydrogen abstraction and OH. addition channels are considered and branching ratios are obtained. Tunneling corrections are taken into account for abstraction channels, assuming unsymmetrical Eckart barriers. The aldehydic abstraction is by far the most important reaction channel within the entire range of temperatures studied, especially at room temperature and lower—the temperatures relevant to atmospheric chemistry. The relative importance of all the other possible channels increases slightly with temperature. Branching ratios show that addition at the ring and abstraction of an ortho hydrogen contribute about 1 % each at about 300 K, while the branching ratio for the main reaction decreases from 99 % at 200 K to 93 % at 350 K. The results are compared with available experimental measurements.