AbstractA quantitative measure,πΔ, for expressing the π‐deficiency and π‐excessiveness of heteroaromatic compounds has been developed through13C NMR chemical shift studies. An example which correlates electrophilic reactivity in π‐excessive systems withπΔ of these compounds is given. The following decreasing order of π‐excessiveness and increasing π‐deficiency of a number of heteroaromatic systems has been established (numbers in parentheses correspond toπΔ): tellurophene (1.83), selenophene (1.43), thiophene (1.35), furan (1.31), benzene (1.00), pyrazine (1.00) pyridine (0.89), pyrimidine (0.77), pyridazine (0.71), 1,2,4‐triazine (0.64). These correlations also allow the prediction of13C chemical shifts in substituted heteroaromatic compounds, where the ratio of the difference between a given substituted heterocyclic compound and its parent, and the identically substituted benzene derivativ