{In this work, a theoretical study is presented on the mechanism of OH reactions with C1-C5 aliphatic aldehydes. We have shown that, starting from butanal, the C beta H-abstraction channel becomes relatively important and it contributes moderately to the total rate constant. Calculated overall rate coefficients at the CCSD(T)/6-311++G**//BHandHLYP/6-311++G**} level are in excellent agreement with experimental data, supporting the proposed mechanisms. Negative activation energies are found to be in agreement with the temperature dependence observed for aldehydes. The branching ratio between the aldehydic and C beta hydrogen abstraction is not significantly modified as temperature increases from 230 to 330 K.