The paper discusses measurements of ammonia (NH3) emissions from EPA's light-duty test vehicle while operated on a dynamometer. The vehicle's (1993 Chevrolet equipped with a three-way catalyst) emissions were measured for three transient (urban driving, highway fuel economy, and hard acceleration) cycles and steady state operation. Previous research showed that NH3 is predominantly emitted from vehicles with a catalyst (three-way or dual-bed). The normal operation of the vehicle's catalyst is to reduce nitrogen oxides (NO-x) to nitrogen (N2) and oxygen (O2). The reduction of NO-x to NH3 emissions were measured during fuel-rich/reduced-O2 conditions (open-loop control scheme), and the results indicated that NH3 production is correlated to combustion conditions. The results also indicated that the amount of NH3 produced correlations with the amount of time that the vehicle remains in the open-loop control scheme. The significance of this finding is that NH3 production can be predicted for a fleet based on the frequency of enrichment of vehicles equipped with a three-way catalyst. The results also provide a way to determine the location of roadway links and/or specific locations where NH3 production can be anticipated based on predicted engine power.