To explore the importance of delocalization on the reactivity of small rings, activation and reaction enthalpies (∆H ‡ and ∆Hr) were calculated for the addition of methyl radical to a test set of 12 acyclic, monocyclic, and fused polycyclic hydrocarbons with ring sizes varying from three to five (Figure ). Applying the BEP principle (eq 2) to this set revealed that, as anticipated, ∆Hr alone is an inaccurate predictor of reactivity (Figure ), with a poor correlation (R 2 = 0.51) and a root mean squared error (RMSE) of 10.1 kcal mol -1 . Particularly notable is the >30 kcal mol -1 span of activation enthalpies for [1. 1.1]propellane (H), cyclopropane (B) and cyclobutane (C) (∆H ‡ = 5.0, 26.4 and 36.1 kcal mol -1 , respectively) despite their similar reaction enthalpies (∆Hr = -28.2, -28.4 and -26.8 kcal mol -1 ). However, incorporation of bond delocalization (𝜒 = 2-Nocc) into the model (i.e. eq 3) resulted in an excellent correlation between predicted and calculated activation enthalpies (Figure , R 2 = 0.97) and low RMSE (2.5 kcal mol -1 ). The negative value of the 'delocalization coefficient' 𝛽 (-192 kcal mol -1 e -1 ) reflects the decrease in the intrinsic barrier with increasing delocalization. Inclusion of the 2-Nocc parameter alongside ∆Hr 2 leads to near-identical results (Figure ). Employing density-based delocalization parameters was similarly successful in predicting activation barriers (R 2 = 0.94, RMSE = 3.3 kcal mol -1 , Figure ), independently validating the interpretation that the localized NBO descriptor effectively captures the electron delocalization effect. Notably, descriptors based on canonical orbital properties (e.g., HOMO-LUMO gap) gave unphysical results (SI Figures ), such as negative intrinsic activation barriers. These results not only confirm that our model improves the originally poor correlation obtained by the BEP principle, but also provides a physically-grounded explanation of the connection between 𝜒 and electron delocalization, as illustrated by these orbital and density analyses.