This paper describes the results of an ab initio investigation on the hydrogen abstraction reaction between CXnY3−nCHO (X = F, Y = Cl and n = 0–3) and OH radical. Equilibrium geometries and transition-state structures have been determined at the (U)MP2/6-311G(d,p) level. The barrier heights and heats of reaction for the hydrogen abstraction reaction have then been estimated from the single point calculations at the QCISD(T)/6-311G(d,p) level. The rate constant for hydrogen abstraction has been calculated by using the standard transition state theory, taking into account the effect of tunneling across the barrier.