The amount of the waste ion-exchange resins are increasing, therefore, the effective method to reduce the volume of the waste resins, is required. The purpose of this paper is to reduce the weight and volume of the resins by reactive thermal plasmas, and to investigate the reaction mechanism of the resins doped with Co and Cs. In this experiment, cation exchange resins doped with Co and Cs were used as sample resins. The resins were treated by thermal plasmas under atmospheric pressure. The weight was reduced to approximately 51% after 60 min treatment without oxygen injection. At 181/min of oxygen flow rate, the weight was reduced to approximately 91%. Small difference in the weight reduction was found among the undoped resins and Co and Cs doped resins. Thermal plasma treatment provides the effective and rapid reduction in weight and the volume of resins. The treatment has good advantages for the stabilization of the doped metals. (C) 2003 Elsevier Science B.V. All rights reserved.