Hexahydrophtalic anhydride is studied by means of a simultaneous thermal analyzer (STA) over a temperature range of 20-400 degreesC, at the heating rates of 5, 10, 20, and 40 degreesC/min. All the experiments have been conducted under argon gas environment and then repeated with dried air as the purge gas. All samples show the melting transition followed by the vaporization indicated by the weight loss. The activation energy calculated from the weight loss data has been compared with the enthalpy of vaporization. Although a zero order transition fits well with a reaction order model, the kinetics of vaporization does not follow this model.