Nitroimidazole exhibits a remarkable regioselective fragmentation subsequent to valence ionization, which is characterized by ejection of NO. As NO is also considered to be an effective radiosensitizer, we investigated its production efficiency as a function of isomeric composition (the site of the NO2 nitro group). We observe strong dependence in the 8.6-15 eV binding energy range, and moreover, that the production of NO can be effectively suppressed by methylation of nitroimidazole. This behavior can be understood by modification of the valence electronic structure with respect to the dissociation threshold, which gives rise to varying effective density of dissociative states. We find the NO yield to follow the efficiency of the nitroimidazole dervivatives as radiosensitizers, found in preclinical studies.