Tetrachloro-m-xylene (tetrachlorometaxylene, or TCMX) is the organochlorine compound with the formula C<sub>6</sub>Cl<sub>4</sub>(CH<sub>3</sub>)<sub>2</sub>. It is the chlorinated derivative of m-xylene in which the four aromatic hydrogen atoms are replaced by chlorine. It is prepared by ferric chloride-catalyzed reaction of the xylene with chlorine.
TCMX is used as an internal standard in the analysis of organochlorides, particularly organochloride pesticides.