The very specific operating conditions of catalytic processes for the abatement of pollutants from mobile or fixed sources, require an unusual experimental set-up to determine the catalyst performance. Selected examples of environmental catalysis applications such as deNO(x) with NH3 or hydrocarbons, deSO(x) with either powder or monolith catalysts, low temperature soot oxidation, are discussed with respect to the experimental methods to be employed for catalytic activity measurements.