This paper describes the results of a study that examined the mechanism of phased perturbation as an emissions control technique. Phased perturbation involves independently controlling the fuel delivered to each bank of a dual bank engine (or each cylinder of a single manifold engine), which allows the two banks to have an adjustable, relative Air/Fuel (A/F) perturbation phase-shift from one another. The phase shifted exhaust is then recombined to achieve a near stoichiometric mixture prior to entering a single underbody catalyst. Phase shifting the exhaust Air/Fuel ratio creates a situation in which both rich exhaust constituents (unburnt hydrocarbons and carbon monoxide) and lean exhaust constituents (oxygen and oxides of nitrogen) arrive at the catalyst at the same time.The results of the study showed that phased perturbation produced a significant effect on A/F control and catalyst THC, CO, and NOx efficiency. The magnitude of the effect of phase shift on conversion efficiency was comparable to the effect of both frequency and amplitude. Due to the magnitude of the effect of A/F phase shifting on catalyst efficiency, it has been identified as a third dimension of A/F control for optimization of exhaust composition as it enters the catalyst. The development of this technique could augment or replace EGR for NOx control (particularly at high catalyst space velocity conditions and during cold engine operation when EGR may not be desirable), may greatly improve overall CO efficiency, and may help reduce cold-start vehicle emissions.