The thermomechanical properties of composites on polymer matrix at high tem­ peratures are essentially different from those at normal temperatures. The main distinctions briefly consist in the following: • at high temperatures there occurs an irreversible variation (degradation) of all mechanical and thermal properties of a material that usually has a complex non-linear character depending on time exposure under high temperature; • there are complicated internal physico-chemical processes in a matrix and fibres under high temperatures called by the general notion of ablation; the internal physico-chemical and mechanical processes run differently in the matrix and fibres, and this leads to the appearance of considerable internal thermal stresses. Generally speaking, a composite under high temperatures can be considered as a multiphase system consisting of solid, gaseous and fluid phases interact.ing mechanically and chemically with each ot.her. There are t.hree levels of temperature: normal, elevated and high. Normal, or room temperatures are 10 - 30°C; elevated temperatures are 30 200°C; hzgh temperatures are those above 200°C. However, the dividing line between elevated and high temperat.ures depends on the material involved; a temperature is called high for a particular composite material if, at this temperature, irreversible internal physico-chemical transformations occur in the matrix and/ or fibres of the material.