This invention concerns to a process for .alpha.,.alpha.,.alpha.,.alpha.',.alpha.',.alpha.'-hexachloroxylene by reacting m-xylene or p-xylene or a partially chlorinated product thereof with chlorine under the irradiation of ultraviolet rays, the process comprising (1) a first-step chlorination reaction which is carried out in the presence of .alpha.,.alpha.,.alpha.,.alpha.',.alpha.',.alpha.'-hexachloroxylene as a solvent added at the initial stage of the reaction, and continued until a compound convertible to .alpha.,.alpha.,.alpha.,.alpha.',.alpha.',.alpha.'-hexachloroxylene by chlorination becomes substantially absent in the reaction mixture, and (2) a second-step chlorination reaction which is continued from the first-step chlorination and carried out to convert difficulty-separable by-products present in the reaction mixture to easily-separable compounds. This invention also concerns to a process for producing an aromatic dicarboxylic acid chloride which comprise reacting the resulting .alpha.,.alpha.,.alpha.,.alpha.',.alpha.',.alpha.'-hexachloroxylene with isophthalic acid or terephthalic acid.