Bidirectional composites on the basis of either plain-weave or satin-weave glass fabric and two types of polysiloxane were cured and then subsequently pyrolyzed at 400, 500, 600 and 700 degreesC in nitrogen. Flexural strength, Young's 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 infrared and Raman spectroscopy. It was found that the composites carbonized at 500 and 600 degreesC, i.e., in the transition domain between the polymer and the ceramic 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 dry air at 400 and 700 degreesC were performed only with samples carbonized at these two temperatures. (C) 2003 Published by Elsevier B.V.