In the decomposition of perchloroethylene (PCE) by gamma-rays, clay from tidal Hat sediments showed an efficient catalytic activity. Moreover, the clay thermally treated at 700 degrees C in air enhanced the PCE removal efficiency better than that of the well known catalyst Degussa P-25 TiO2. The change of the clay by the thermal treatment was identified by electron paramagnetic resonance {EPR) spectroscopy. The intensity of the signal that arises from the natural radiation defect was decreased with increasing thermal treatment temperature, and inversely dependent on the PCE removal efficiency.