In the present study, a kinetic analysis on decomposition processes of some oxacillin salts (oxacillin, cloxacillin and dicloxacillin) was carried out to identify their kinetic parameters. As expected by their complex structures, several steps with different activation energies occurred in the decomposition processes. The application of the model-fitting kinetic method to multi-step decomposition process results to be unsuitable for the non-isothermal data. As far as isothermal data are concerned this method gives rises to more reliable results which, however, are likely to conceal the kinetic complexity. The model-free approach represented by isothermal and non-isothermal isoconversional methods, gives different dependencies of the activation energies as a function of the extent of conversion and allows detecting multi-step processes over a wide temperature range.