The deactivation and regeneration of selective catalytic reduction (SCR) catalysts poisoned by potassium by a wet-impregnation method was investigated experimentally. Potassium in the form of both chloride and sulfate is a strong poison for the catalyst. The results indicate that potassium titrates the active sites for NH3 adsorption. Simply increasing the operating temperature or the vanadium content in the catalyst cannot compensate the loss of catalyst activity: Increasing the temperature hardly increases the conversion of NO for the strongly poisoned catalysts, and catalysts with high vanadium content become active for oxidizing NH3 to NO, causing a net NO formation. Deactivated catalysts can be regenerated by different methods. Sulfation by gaseous SO2 is efficient provided the poison is first removed by washing. When regenerating by 0.5 M H2SO4, the catalyst regains a higher activity than that of the fresh catalyst at temperatures higher than 300degreesC. Heat treatment of the catalyst at 400degreesC for 2 h after poisoning to simulate actual operation has no influence on the regeneration by 0.5 M H2SO4. Deactivated catalysts without the heat treatment step regain higher activities than that of the fresh catalyst at all temperatures when regenerated by 1 M NH4Cl. However, the heat treatment step has a negative effect on the regeneration by NH4Cl.