The control loop designed to maintain a stoichiometric A/F ratio for the efficient operation of three way catalysts used in automotive catalytic converters imposes a 0.5 to 1 Hz modulation onto the exhaust composition reaching the converter. Although many laboratory studies show composition modulation can lead to large increases in conversion, these increases are not seen for catalytic converters. The operating temperatures, 400 to 800 degrees C, seem to be the reason. Analysis of published data suggests rate enhancement through composition modulation decreases with increasing temperature if reactant inhibition is present. The optimal cycling frequency increases with temperature so that the control loop induced modulation is sub-optimal for normal converter operation.