7-Chloro-3-[[N,N-dimethylamino)propyl]amino]-2-quinoxalinecarbonitril e 1,4-di-N-oxide hydrochloride (Q-85 HCl), a new hypoxia-selective agent, was studied to clarify its physico-chemical properties. Melting point, spectra (UV, IR, NMR, MS), thermal behaviour, solubility, pKa, partition coefficient and chromatographic characteristics (TLC, HPLC) have been determined. The stability of Q-85 HCl was studied under various conditions by HPLC. Q-85 HCl in solid state was very stable to heat and fairly stable to light. Some degradation products of Q-85 HCl were identified in basic solution.