approach to the catalyst surface through a-complex adsorption suggests that the main cause for the faster aromatic exchange is due to a substitution process. The dissociative r-complex substitution mechanism is further supported by the observed "complete" and "severe" ortho deactivation effects.The production of nitrite from neutral and alkaline solutions of sodium nitrate by Coeo y-radiation was measured over a range of nitrate concentrations from to 4.0 M . The production of 0 2 from these solutions at pH 13 was also measured, and the origin of the 0 2 was determined by labeling the water with 0'8. The effect of scavengers, particularly 0 2 and I-, on the nitrate reduction was determined. From these studies, and from comparisons with ultraviolet photolysis and with previously published work, the radiolysis of NO3in neutral solutions was concluded to involve three effects: (1) reduction of NO3by electrons and H atoms, (2) direct excitation by radiation, and (3) an effect possibly due to excited water molecules or spur reactions. In alkaline solutions an additional contribution is found, probably from the reaction of 0-. The data allow a determination of G,,,-2 3.8 and G-H~O 4.5 at pH 13.(1) Work performed under Contract No. AT(07-2)-1 with the U. S.