Modified acid treated bentonites have been studied as catalysts for the removal of nitrogen oxides (NOx) from power plants off-gases in the selective catalytic reduction of NOx by ammonia (SCR-NH3). The main steps of this work have been the preparation of samples, characterization of samples (by XRD, FT-IR, UV-vis and TG) and the application of obtained catalysts in SCR-NH3. Clays and clay minerals are recognized as the materials of the 21st century for being abundant, inexpensive and environmentally friendly; therefore, they appear to be good candidates for their application in SCR to overcome the limits of commercial catalysts. Modified bentonites have been here investigated: acid treatment and pillaring resulted to be the best methods of modification of these clays.