Spent low-level radioactive (LLRA) cation ion exchange resins (consisting of base copolymers of styrene and divinyl benzene and sulfonic acid) are difficult to treat effectively by conventional cement-solidification methods. Pyrolysis of the spent LLRA resin has been recognized to be very effective in reducing the volume and mass of the waste. Experimentally, we found by the thermogravimetric analysis (TGA) method that the activation energy for the pyrolysis of an LLRA resin was 319.2 kJ/mol. The reaction order and pre-exponential factor were 0.61 and 1.1 × 1020 s-1, respectively. Note that during the pyrolysis, ~50% of the SO42- species in the resin was decomposed to SO2 at 673 to 873 K. At high temperatures (>873 K), most of the aromatics of the resin were also thermally cracked.