Thermal stability of sulfonamide potentiator-trimethoprim is studied by thermogravimetry and differential thermogravimetry. One weightloss phase is observed in the heating courses from room temperature to 700 °C in the inert atmosphere. Using Kissinger and Ozawa methods, the activation energy of thermal decomposition is obtained as 70.2276 KJ/mol. The kinetic equation of thermal decomposition can be expressed as dα/dt = 1.5238 × 10 5 ) RT 78794 exp(-(1 -α)[-ln(1 -α) 0.5057 ].