Ceria is a common component of engine aftertreatment catalysts due to its oxygen storage ability, its redox properties, and its role in stabilizing Pt against sintering. The interactions between ceria and NH3 or NO x were investigated to better understand the role of ceria in oxidation reactions occurring over a diesel oxidation catalyst, in the reduction of NO x on lean NO x traps, and in the selective catalytic oxidation (SCO) of NH3. Ceria proved active in NO oxidation, selective catalytic reduction of NO by NH3 (NH3–SCR), and NH3–SCO reactions. Between 100 and 450 °C, both NH3 and NO x adsorbed on ceria simultaneously. In the absence of NO x , NH3 was oxidized over CeO2 forming N2 via a two-step selective catalytic reduction mechanism at low temperature and NO x at high temperature. In the presence of NO x , NH3 reacted with adsorbed NO x species, again forming N2 at lower temperatures (250–450 °C), while at higher temperature, a significant portion of the NH3 was oxidized, with product NO formed.