Only electronic factors are responsible for the kinetics and mechanism of consecutive chlorination of 2,3,7,8-tetrachlorodibenzo-para-dioxin and 2,3,7,8-tetrachlorodibenzofuran; this results in the formation of a standard quasi-stable mixture of chlorination products (congeners), polychlorinated dibenzo-p-dioxins and dibenzofurans, whose composition was independent of the sample type and the place of sampling.