Oxidation catalysts are often utilized to reduce carbon monoxide, formaldehyde, and volatile organic compounds in order to meet emissions regulations for large bore natural gas engines. These catalysts degrade over time and need to be replaced or regenerated to maintain emissions compliance. This work evaluates the effectiveness of catalyst regeneration, or catalyst washing. The evaluation is performed by utilizing field and laboratory slip streams combined with catalyst module performance (reduction efficiency) measurements and catalyst material surface analysis to quantify catalyst poisons.