In the present study, the kinetic parameters of the thermal decomposition of ulexite were investigated by using TGA data. For the kinetic analysis, the Suzuki and Coats-Redfern methods were applied. It was determined that the process fits a first-order kinetic model, and the value of the activation energies and frequency factors decreased with decreasing particle size, which can be attributed to the increasing particle internal resistance to the escape of water as the grain size increases. The activation energy values were found to be 47.34-60.01 kJ mol(-1) for region I and 0.225-1.796 kT mol(-1) for region II for the range of particle size fraction used. The frequency factors were calculated to be 9821.8-524.9 s(-1) for region I and 3.05x10(-4)-2.807x10(-5) for region II for the same conditions.