AbstractThe reaction of the 2,4,5‐trichlorophenolate anion with 2,4‐dichlorophenol to afford trichlorinated dibenzo‐p‐dioxins (T3CDDs) is investigated at the B3LYP/6‐31+G(d) and B3LYP/6‐311+G(3df,2p)//B3LYP/6‐31+G(d)+ZPVE(B3LYP/6‐31+G(d)) levels of theory. The first stage of the process corresponds to the formation of a predioxin, which can evolve through four different routes. Two of them lead directly to the products 2,3,7‐T3CDD and 1,3,8‐T3CDD, and the other two afford different predioxin‐type intermediates, which in turn can evolve through all or some of the four routes to give new predioxins or T3CDD. Consequently, the theoretical results obtained show plainly the complex chemistry implied in the formation of dioxins from chlorophenols via anionic mechanisms by disclosing all the critical structures through which the system evolves, thus allowing assessment of the viability of the different mechanistic routes and the accessible products. The statistical thermodynamics treatment at