The equilibrium NH acidities of acridan, phenanthridone, 9-acridone, and 2-substituted 9-acridones (substituents: Me2N, MeO, F, Br, I, and NO2) were measured by transmetallation in dimethyl sulfoxide (DMSO). The role of conjugation and the aromatic character of the heterocyclic ring in stabilization of the anions is discussed. The energies of deprotonation of phenanthridone and 9-acridone were estimated by the CNDO/2 (complete neglect of differential overlap) method; it is shown that the difference between them is in agreement with the experimental results. The dependence pK = 16.2 − 3.37σI − 1.84σR (s 0.23, r 0.989) was obtained for the 2-substituted 9-acridones.