A NOx trap catalyst was evaluated in a light-duty diesel engine bench under steady-state speed/load conditions with alternating lean and rich exhaust streams. The NOx conversion was correlated with several engine operating and control parameters, such as speed, lean / rich timing and catalyst temperature. The NOx conversion is a result of balance between stored NOx in a lean stream and the quantity of reductant applied in a rich transient pulse. The conversion is inversely proportional to the lean / rich ratio, R, (at R< 17) and engine speed. At a given speed and lean/rich ratio, the conversion is proportional to the catalyst inlet temperature. If the temperature is too high, thermal NOx release may decrease the overall NOx conversion. With a fully regenerated NOx trap catalyst, its cumulative NOx storage, at a given trapping period (or an instantaneous NOx trapping efficiency), is proportional to engine speed. It is believed that the catalyst inlet temperature is the most important factor in determining the NOx trapping capacity. The findings of this work are discussed in light of the underlying chemical principles of the NOx trap processes and are expected to provide some guidance on the engine control strategy related to the application of NOx trap technology.