AbstractThe13C NMR spectra of nine dichlorinated bicyclo[2.2.1]heptanes (norbornanes) have been measured and assigned. The pairwise effects of chlorine substituents which cause deviations from the additivity of single‐substituent effects were investigated and are discussed. The largest effect found is the high‐field shift of carbons bearing vicinalcissubstituents. In the case of geminal substitution deviations from additivity were found to be to low field and large in the γ, smaller in the β and negligible in the α chemical shifts. The observed deviations for 1,3‐disubstituted cases vary from −3.2 to +1.1 ppm at different carbons, allowing no simple explanation. Replacement of α‐hydrogen in a diaxial 1,3‐arrangement by CH3, OH or CI causes the single substituent effect, namely the γaeffect, to cha