Steric acceleration by α,β-polycyclic fusion in pyridine leaving groups is quantitatively assessed. Constraining phenyl rings in pyridines to near planarity by ethano bridges in phenanthro[2,3-h]quinolinium (39) is less effective in terms of SN2 displacement reactions by a factor of ca. 20 than in the less sterically hindered benzoquinolinium (2b). However, the corresponding rate for diphenanthro[2,3-c;3′,2′-h]-acridinium (43) is almost the same as that for the dibenzoacridinium (3b) systems. The pendant phenyl groups in systems (12) and (19) decrease SN2 rates compared with (2a) and (3a) by factors of ca. 2 and ca. 4, respectively. Neither of these structural modifications significantly increases SN1 rates.