Ionization (IP) and appearance potentials (AP) have been secured for apparently competing [M - NO,] cleavage and [M -NO] rearrangement reactions in a series of mand p-X substituted nitrobenzene molecular ions. Approximate activation energies AP-IP were within the experimental reproducibility for most substituents for the [M -NOz] reaction of m-and p-X isomer pairs of compounds, but outside it for the [M -NO] process. This is interpreted as either effective substituent randomization in the reacting molecular ions or as fortuitously similar activation energies for the [M -NO,] cleavage. Assuming the latter, AP-IP for both reactions gave acceptably linear correlations of negative slope when plotted against u or u+, which has been interpreted mechanistically in terms of a degree of positive charge dissipation in the rate limiting transition states. These findings * The approximation is due to the kinetic shift, to excess energy problems, and to instrumental errors and extrapolation procedures used to obtain IPS and A P ' S . ~* J ~~~~ t For X = OCHI, the [M -301 peak was shown by precise mass measurements to be t 9 7 % [M -NO], rather than [M -CH,O]. For X = NOz, AP-IP for [M -NO] was less than the reproducibility of the determinations (i.e. t 0 . 2 ev>.