The substance of the project was to design a oxygen-resistant matrix on the basis of modified siloxanes and to use suitable glassy fabrics for preparation of heat resistant composites.Bidirectional composites on the basis of either plain-weave (E-glass) or satin –weave fabric (R-glass) and two types of polysiloxane (Lukosil 901, Lukosil 130) were cured and subsequently pyrolyzed at the temperatures 400,500,600 and 700°C in nitrogen. Flexural strength, Young elasticity modulus and shear modulus before and after oxidation were determined. Structural changes of the matrix in the course of carbonization were identified with microscopy and with IR and Raman spectroscopy. It was found that the composites carbonized at the temperatures 500 and 600°C, i.e., in the transition domain between the polymer and the cetramic matrix, exhibit the lowest values of the mechanical parameters as in this range a significant loosening of the matrix takes place. For this reason, oxidation tests in the stream of dray air at the temperatures 400 and 700° were performed only with samples carbonized at these two temperatures.The probes remain compact after oxidation, but in the case of E-glass, deformation already takes place at the maximum temperature.