The method of electron spectroscopy entailing quantum-chemical calculations and the IR absorption spectra demonstrated the existence of the mono-, di-, and triprotonated forms of anthrapyrimidine and its derivatives in acidic solutions. The order of the protonation of the anthrapyrimidine molecule was established. It was shown that the two nitrogen atoms add protons; the third proton binds with the oxygen atom of the carbonyl group. In the molecule, the positive charge is localized within the pyrimidine ring. The energy characteristics of the anthrapyrimidine and its protonated forms were calculated. © 1991 Plenum Publishing Corporation.