The kinetics and mechanisms for the unimolecular dissociation of nitrobenzene and related association reactions C6H5 + NO2 and C6H5O + NO have been studied computationally at the G2M(RCC, MP2) level of theory in conjunction with rate constant prediction with multichannel RRKM calculations. Formation Of C6H5 + NO2 was found to be dominant above 850 K with its branching ratio > 0.78, whereas the formation of C6H5O + NO via the C6H5ONO intermediate was found to be competitive at lower temperatures, with its branching ratio increasing from 0.22 at 850 K to 0.97 at 500 K. The third energetically accessible channel producing C6H4 + HONO was found to be uncompetitive throughout the temperature range investigated, 500-2000 K. The predicted rate constants for C6H5NO2 → C6H5 + NO2 and C6H5O + NO → C6H5ONO under varying experimental conditions were found to be in good agreement with all existing experimental data. For C6H5 + NO2, the combination processes producing C6H5ONO and C6H5NO2 are dominant at low temperature and high pressure, while the disproportionation process giving C6H5O + NO via C6H5ONO becomes competitive at low pressure and dominant at temperatures above 1000 K.