The oxidation of highly porous ceramic matrix composites (PCMCs) based on different Tyranno((R)) fibers has been analyzed by means of thermogravimetry and electron microscopy. Both uncoated fibers and PCMC materials exhibit parabolic kinetics between 900 degrees C and 1250 degrees C, these being faster for Ti-doped than for Zr-doped Tyranno fibers. Oxide layers in Ti-doped fibers are porous and partially crystalline, whereas in Zr-doped materials a significant fraction of relatively coarse -SiC grains is still found embedded in the amorphous silica matrix. On the other hand, the CVD-SiC coatings exhibit higher oxidation rates from the outer surface than from the inner one, a phenomenon that has been associated not only with the more difficult access of oxygen to the inner face but also with the highly < 111 > textured structure of these coatings, for which very different oxidation rates have been published for the inward and outward directions. Cracking phenomena observed above 1100 degrees C for long dwelling times do not lead to an acceleration of the oxidation process, which could be due to the simultaneous crystallization of the amorphous silica layers.