On the basis of IR and electronic spectra and quantum-chemical calculations, a conclusion has been drawn regarding the structure of phenanthridone and its derivatives: It has been confirmed that in the crystalline state and in solutions, phenanthridones exist primarily in the lactam form. An interpretation is given for the longwave bands in the electronic absorption spectra. Certain spectral criteria are proposed for determining the positions of substituents in the phenyl rings of phenanthridones, and the direction of aromatic substitution of the lactim and cationic forms of phenanthridone have been evaluated. Acid association constants of phenanthridone and its derivatives have been determined. © 1993 Plenum Publishing Corporation.