There is provided a process for mononitrating ortho-xylene by reacting o-xylene with nitrogen tetroxide (N2O4) or nitrogen dioxide (NO2) and an oxygen containing gas, such as oxygen, in the presence of a mercuric salt dissolved in acetic acid to obtain good yields of mononitrated o-xylene having an isomer distribution of at least a 70:30 ratio of 4- to 3- nitro-oxylenes. Such 4-nitro-o-xylene, upon separation from the isomeric mixture, finds utility in the preparation of xylidine derived preemergence herbicides.