Methane, hydrogen chloride and oxygen at high temperatures yield a mixture of chlorinated alkanes, -alkenes, -aromatics as well as polychlorinated dibenzo-p-dioxins (PCDD's) and polychlorinated dibenzofurans (PCDF's) containing up to 3 chlorine atoms. The pattern of products formed is similar to the distribution of chlorinated micro-pollutants found in flue gases from municipal waste incinerators. Dibenzo-p-dioxin and dibenzofuran react with hydrochloric acid at 400°C in sealed ampoules. The products are the components of the high temperature equilibrium reactions of chlorinated alkanes besides PCDD's and PCDF's. These results suggest that PCDD's and PCDF's are formed in a chain of reversible reactions starting with chloromethanes.