Secondary air scheduling and average delivery rate have a great influence on the performance (carbon monoxide and hydrocarbon cleanup) of rich thermal manifold reactors. A continuously modulated secondary air system was devised to provide a tailpipe air-fuel ratio that did not change significantly with engine speed or load when a “flat” carburetion calibration was incorporated. This system involved throttling the inlet of the air pump(s) so that the air pump and engine intake pressures were equal. The continuous air modulation system was compared with an unmodulated system and a step-modulated system. The secondary air systems were investigated with both GMR “small volume” cast iron thermal reactors and Du Pont V thermal reactors on modified 350 CID V-8 engines in 1969 Chevrolet passenger vehicles.It was found that thermal reactor performance improved with each increase in control of the secondary air schedule. With the continuous air modulation system a reduction in CO emissions of approximately 45% (on the 1972 Federal Test Procedure emissions test) was achieved relative to an unmodulated system. None of the systems tested, however, demonstrated the capability to achieve 1975-1976 Federal exhaust emission standards.