The reactivity of the thienyl diazines 1 to 7 has been studied by means of the nitration reaction. In order to examine any relationship between the reactivity and the electronic structure of these compounds, we have recorded their 13C nmr spectra in CDCl3 and in concentrated sulfuric acid.The 13C results are correlated to relative reactivities observed during competitive reactions between the thienyl diazines 1 and 2, 2 and 7, 2 and 4, 2 and 6.This method is a rarely-used approach for the study of electrophilic substitution of a molecule in the reaction mixture.