Recent internal research performed at SwRI examined an emissions control mechanism that we have labeled, ‘phased A/F perturbation.’ The suggested mechanism of phased perturbation involves independently controlling the fuel delivered to each bank of a dual bank engine, which allows the two banks to have an adjustable, relative A/F perturbation phase-shift from one another. Exhaust from the two banks can be combined to achieve a near-stoichiometric mixture prior to entering a single underbody catalyst. Since both rich and lean exhaust species would be present simultaneously, a highly reactive mixture would continuously enter the catalyst.In that work, it was found that A/F phasing produced as significant an effect on conversion efficiency as perturbation amplitude and frequency, i.e. A/F phasing was identified as a third dimension for optimization of exhaust gas composition as it enters the catalyst. It was also observed that A/F phasing created conditions under which the catalyst conversion efficiency became insensitive to variations in engine-out A/F perturbation frequency and amplitude. This was very important, because under standard fuel control, a catalytic converter typically uses it's oxygen storage capability to help reduce the effects of A/F perturbation frequency and amplitude on conversion efficiency. These results indicated that a 180° phase shifted exhaust may not heavily rely on oxygen storage to complete the catalytic reactions.This paper examines the interaction between A/F phase shift and catalyst washcoat ceria (an oxygen storage element) content, and their subsequent effect on catalyst conversion efficiency. Three catalysts with varying levels of ceria (a standard level, 50 percent less, and zero percent ceria) were evaluated. On each catalyst, the effect of A/F phase shift on the conversion efficiency was measured using A/F sweeps. In addition, the effects of perturbation amplitude and phase shift on NOx efficiency were examined on the lean side of stoichiometry.