Recently it has been noted that when o-xylene is treated with nitric acid in acetic anhydride, in addition to the expected 3- and 4-nitro-o-xylenes, 3,4-dimethylphenyl acetate is formed and is the major product. To follow the kinetics of the simultaneous nitration and acetoxylation reactions the nitroxylenes were separated from the acetoxylated derivative and the concentration of each solution was then determined by ultraviolet absorption measurements. The investigation of the concurrent reactions has led to the identification of the reactive electrophile. The rates of both the nitration reaction and of the acetoxylation reaction for o-xylene have been shown to be third order in nitric acid concentration and zeroth order in substrate concentration. Furthermore, the addition of sulphuric acid or acetic acid to the reaction mixtures accelerates the rates of those reactions, while in the presence of added lithium nitrate they are retarded. Most important, the ratio of the rates of acetoxylation and nitration remains constant under all reaction conditions. This evidence is accounted for in terms of one reactive species, protonated acetyl nitrate, which leads to both acetoxy and nitro derivatives. Earlier kinetic data from the literature in reinterpreted in terms of this species.