The structures of 5-hydroxy and 5-amino derivatives of naphtho[2,3-h]quinoline-7,12-dione (anthraquinonepyridine) and their conjugate acids were investigated by experimental and computational [Pariser-Parr-Pople (PPP)] methods. The hydroxy derivative exists in the keto form, while the cation of the hydroxy form is formed during protonation; the amino derivative exists in the amino form but is converted to a cation with an imino structure upon protonation. In both cases the addition of a proton is accompanied by rearrangement of the π-electon structure of the molecules. The assignment of the Sππ* transitions in the electronic spectra of the bases and their conjugate acids is given on the basis of a quantum-chemical calculation. © 1981 Plenum Publishing Corporation.