The ratio of isomeric bisperchloratomercuribenzenes obtained from the monomercurial and mercuric perchlorate in perchloric acid varies slightly with concentration of the acid, but is preponderantly meta. However, more para than meta isomer is obtained under conditions where the para isomer is insoluble in the system. This indicates a mobile equilibrium among the isomers, which is found to extend to all of the polymercuration products. The postulated intermediate for the equilibration, which is 1,2,4-trichloratomercuribenzene, is indicated by the isolation of the similarly oriented 1,2,4,5-perchloratomercuritetrakisbenzene. Although none of the ortho-diperchloratomercuribenzene can be found from this reaction in perchloric acid, it is the principal isomer when phenylmercuric acetate is acetoxymercurated neat.