9(10H)-Acridinone and its 13 derivatives were examined at the semiempirical (MNDO, AM1, PM3, MNDO/ d), ab initio (HF, MP2), and density functional (DFT) levels of theory with the 6-31G**, 6-31++G**, and 6-311G** basis sets to predict structure, enthalpies and free enthalpies of formation, entropies, HOMO and LUMO energies, and dipole moments. The possibilities of some of the compounds occurring in more than one tautomeric (isomeric) form were further considered. The behavior of 9-acridinones and their features, which may be useful if the compounds are to be used as analytical spectral indicators or fragments of fluorescent or chemiluminescent labels, are also briefly discussed.