To compare the OH-initiated reaction mechanisms of five chlorophenoxy herbicides, density functional theory (DFT) calculations of reactions in which ·OH attacks one of three active positions on each herbicide were carried out at the MPWB1K/6-311 + G(3df,2p)//MPWB1K/6-31 + G(d,p) level. For each herbicide, the calculation results show that ·OH addition to the C1 atom, which is the nexus between the benzene ring and the side group, possesses the lowest energy barrier among the three kinds of reactions, indicating that ·OH addition-substitution of the side chain is the most energetically and kinetically favorable reaction mechanism. Comparisons among the herbicides show that the mechanisms are affected by the steric hindrance and the electronegativities of the -CH₃ and -Cl groups. When comparing the addition of ·OH to the C1 site among the five herbicides, the activation energy for the reaction of ·OH with DCPP reaction is the lowest (3.61 kcal mol(-1)), while that for the ·OH and 4-CPA reaction was the highest (5.91 kcal mol(-1)). ·OH addition to the C4 site presents the highest energy barriers among the three kinds of reactions, indicating that the para Cl is difficult to break down. When comparing the H-atom abstraction reactions of the five herbicides, the H atoms in the -CH₂- group of 2,4-D are the easiest for ·OH to abstract, whereas those of DCPP and MCPP are more difficult to abstract, due to the steric hindrance of the -CH₃ group. Additionally, the results obtained from the PCM calculations reveal that most of the reactions occur more easily in water than in gas, though the mechanisms involved are the same as those discussed above.