In a matrix of anthracene (An) and 9,10-dihydroanthracene (AnH(2)) (1:1) between 350 and 400 degrees C in pressurized liquid systems, the rates and mechanisms have been studied for desubstitution of a number of aromatic compounds : naphthyl-X, with X = Cl, Pr, F, D, CH3, NH2, CH=CH2, OCH3, OK, Ph, C(O)CH3. It appears that for these compounds hydrogen transfers via radical hydrogen transfer (RHT) by 9,10-dihydroanthracenyl radicals (9-AnH(.)) or reverse radical displacement (RRD) with 9,10-dihydroanthracene are the major desubstitution pathways. However, for bromo- and chloronaphthalene, desubstitution is much faster and naphthyldihydroanthracenes and naphthylanthracenes are the main products. In these cases Radical displacement (RD) by 9-AnH(.) is the predominant route. Also, condensation between An and naphthyl-X is noticed. During these experiments the hydrogenation of aromatic rings was observed as well : anthracene --> 1,2,3,4-tetrahydroanthracene and naphthalene --> tetralin. This process has been studied by employing naphthalene-d(8). The rate of H/D exchange appears to be twice as fast as hydrogenation in the naphthalene molecule. A thermodynamic and kinetic rationale is presented to explain the change in mechanism as a function of the substituent.