The kinetic analysis of the thermal decomposition process of polymeric materials has great practical significance, especially for determining the stability of the material under processing and application conditions, as well as for predicting useful life of the product. There are different methods of determining kinetic parameters, but today it is generally accepted that the first step is to precisely determine the dependence of activation energy on conversion by one of the isoconversion methods. These methods allow the calculation of activaton energy without knowing the mechanism of the process. Therefore, the aim of this paper is to investigate whether the sample preparation process polymers impact on activation energy values. Dynamic thermogravimetric analysis (TG) showed that the thermal decomposition of poly (ethylene oxide) (PEO) passes through the one degradation step in the 330-350 °C temperature range. Comparison of TG analysis for pure PEO powder with the results of hot pressed samples and casting films from the solution it can be concluded that the preparation of the sample does not affect the thermal stability of the PEO. The dependence of activation energy calculated by the differential Friedman and integral Kissinger-Akahira- Sunose method on conversion is constant for all samples in a broad conversion range, regardless of how the samples are prepared. Samples that have been prepared by hot press or cast from the solution have lower activation energy than the commercial PEO powder