The autocatalytic nature of the reaction Cu2+ + NO3- = CuO + NO2+ in molten sodium nitrate-potassium nitrate equimolar solvent is confirmed. A steady-state assumption applied to the nitryl ion concentration is shown to be more applicable than the rapid equilibrium assumption used by other workers. Evidence is given to support a proposed mechanism which indicates that solid copper(II) oxide enhances the reaction between NO2+ and NO3- ions. This suggests that the catalytic nature of CuO is not specific to the Cu2+ ion and that CuO should catalyze any acid-base reaction in nitrate solvents for which NO2+ ion can be considered an intermediate species. Also other metallic oxides could possibly act as catalyst for such reactions. © 1969, American Chemical Society. All rights reserved.