Ab initio and semiempirical AM1 self-consistent-field (SCF) and configuration interaction (CI) calculations have been carried out for the anthracene reversible arrow 9,10-Dewar-anthracene and 9-tert-butylanthracene reversible arrow 9-tertbutyl-9,10-Dewar anthracene valence isomerization reactions. The reaction enthalpy for the thermal rearomatization reaction in the tert-butyl-substituted compound is found to be -38 kcal/mol in good agreement with the experimental value of -41 kcal/mol. The ab initio SCF optimized geometries compare favorably with experimental X-ray data. Two-dimensional energy surfaces and the reaction path connecting the isomers have been calculated. The barrier for the thermal back-reaction in the case of anthracene is theoretically found to be about 22 kcal/mol, in good agreement with kinetic data. The calculations support an approximate state correlation diagram given in the literature and offer an explanation for the photochemical stability of anthracene with respect to formation of its Dewar isomer.