The formylation of toluene in trifluoromethanesulfonic acid was studied in a batch reactor to determine the effect on reaction rate of increasing the ratio of trifluoromethanesulfonic acid/substrate at constant temperature. The production rate of p-tolualdehyde increased with increasing amounts of acid so that its yield was 99.1% when the acid/substrate was 20 mol/mol (30 min at 298 K), The reaction obeys first-order kinetics in toluene for conversions less than 25%. The rate of formylation increases with increasing CO partial pressures from 500-2000 psig. The reaction rate shows an optimum temperature near 298 K.