The application of thermal methods is a great importance regarding the pharmaceutical problems such as the control of raw materials, the determination of purity, the qualitative and quantitative analysis of drug formulation, tests of thermal stability and compatibility, the determination of kinetic parameters etc. The purpose of a kinetic investigation is to calculate the kinetic parameters and kinetic model for the studied process. The results are further used to predict the system behaviour in various circumstances. A kinetic study regarding the fusidic acid involving active substance’s thermal decomposition was performed under isothermal conditions in a nitrogen atmosphere, for the temperature steps: 260, 270, 280, 290 and 300°C. The thermogravimetry/derivative thermogravimetry data were processed by three differential methods: isothermal-isoconversional, Friedman’s isothermal-isoconversional and isothermal model-fitting. The careful treatment of the kinetic parameters obtained in different thermal conditions is necessary, as well as the choice of the adequate calculation methods.