Kinetic and regioselectivity data are reported for the Gattermann-Koch formylation of m-xylene, 1-methylnaphthalene, and toluene in HF-SbF5 and CF3SO3H-SbF5 as a function of SbF5/substrate molar ratio. The kinetic study for m-xylene formylation in HF-SbF5 provided crucial evidence in favor of intracomplex reaction via a third-order rate equation, [ArH][ArH2+SbF6-][CO], where the formylation electrophile HCO+ is generated by CO protonation by the arenium ion. Dependence of regioselectivity on substrate, superacid, and SbF5/substrate molar ratio showed that high para regioselectivity stems from intracomplex reaction and the observed regioselectivity reflects the ratio between the intracomplex and the conventional reactions. Comparison in regioselectivity between Gattermann-Koch formylation and Friedel-Crafts formylation with use of HCOF suggested that regioselectivity trends do not reflect the nature of the electrophile but the reaction pathway; the Friedel-Crafts formylation also appears to have intracomplex reaction character.