In this thesis, the thermogravimetric decomposition of PEO/ES composites of different compositions was investigated at three different heating rates in a stream of nitrogen. Based on the values of the characteristic temperatures of the PEO/ES composite, the addition of ground eggshells slightly affects the thermal stability of PEO. Using isoconversional Flyn-Wall-Ozawa and Friedman methods, the average activation energy (E a ) of the thermal decomposition of the mentioned composites was calculated. Based on the dependence of the activation energy of thermal decomposition on the conversion (α), it can be concluded that the activation energy in the whole area of conversion depends on the conversion, suggesting that the mechanism is complex. With the addition of eggshells, the average value of the thermal decomposition activation energy is lowered, but the addition does not affect the decomposition mechanism itself. The composite PEO/ES composition 90/10, which contains the largest proportion of eggshells, can be singled out in particular, which is why there is a visible decrease in E a and a change in the dependence of E a on α, which suggests that there is a change in the reaction mechanism. On the basis of the change of the remaining mass in the dependence of the heating rate, the composites of the composition 99/1 and 90/10 are degraded by a different mechanism compared to the composites of the other compositions.