The photodissociation of isotope-labeled m-xylene, 1,3-C6H4CD3CD3, at 6.4 eV under collision-free conditions was studied using multimass ion imaging techniques. The masses of the photofragments were identified, and the fragment recoil velocity distributions were measured. In addition to the major dissociation channels C6H4CD3CD3 --> C6H4CD3CD2 + D and C6H4CD3CD3 --> C6H4CD3 + CD3, the respective photofragments C6H3DCD3, C6H2D2CD3, and C6HD3CD3 and some of their light fragment partners, CD2H and CDH2, were also observed. The results show that a small portion of the excited xylene isomerizes to a seven-membered ring (methylcycloheptatriene), followed by the isotope exchange within the seven-membered ring, and then rearomatizes to a six-membered ring prior to dissociation. The isomerization pathway competes with the direct C-C bond and C-H bond dissociation in the ground electronic state. The significance of this isomerization is that the carbon atoms and hydrogen atoms belonging to the alkyl group are involved in an exchange with those atoms in the aromatic ring.