The natural zeolites chabazite and clinoptilolite, and synthetic zeolites A and X in their ammonium forms were tested by thermodilatometry. Chabazite, A and X zeolites appeared to catalyze ammonia combustion, whereas clinoptilolite did not. The thermal effects relating to ammonia combustion were ascribed either to different heating rates or to different catalytic activity of the various zeolitic substrates. The different ignition temperatures of ammonia measured on different zeolitic substrates were taken to be a measure of the catalytic activity of the zeolitic substrate itself.