The effect of water in trifluoromethanesulfonic acid (CF3SO3H or triflic acid) on the carbonylation of toluene is reported at room temperature (297 K) and constant CO partial pressure (7.68 MPa = 1100 psig). A method has been developed to measure low concentrations of water in triflic acid by H-1 NMR, The NMR signal increased in value linearly with increasing mole percent of water in the superacid, The first-order rate constant for toluene conversion decreases with increasing water content of the triflic acid when all other experimental variables are held constant. The triflic acid hemihydrate [(CF3SO3H)(2):H2O] is inactive for the carbonylation reaction of toluene.