Nitration of benzene containing 91.5 mole % monodeuterobenzene gave nitrobenzene containing 74.9 mole % monodeuteronitrobenzene and w-dinitrobenzene containing 60.9mole % monodeutero-m-dinitrobenzene. Since these are the concentrations expected if there is no hydrogen isotope effect, these studies confirm a previous report based on tracer studies that loss of a hydrogen ion cannot be a rate-influencing step in aromatic nitration. These values could not have resulted fortuitously from hydrogen exchange since nitration of normal benzene under identical conditions except in the presence of deuterosulfuric acid gave normal nitrobenzene and normal m-dinitrobenzene. A sample of the 91.5 mole % monodeuterobenzene lost 16.2% of its deuterium in less than an hour when heated at 50-60° in heterogeneous mixture with concentrated sulfuric acid. A sample of the 74.9 mole % monodeuteronitrobenzene lost none of its deuterium when treated under similar conditions in homogeneous mixture. This difference in ease of hydrogen exchange is in accord with the usual behavior of these molecules toward electrophilic aromatic substitution.