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The ® bre± matrix reaction that occurs during the processing of a Nicalon SiC ® bre± Pyrex glass matrix composite is analysed. Interphases are characterized by means of various complementary techniques: electron di raction, HRTEM, EDX, EELS, SIMS and XPS. Neither the results of the present study nor those previously obtained for Nicalon SiC ® bre± LAS (Li 2 O± Al 2 O 3 ± SiO 2) glass or LAS + MAS (MgO± Al 2 O 3 ± SiO 2 ) glass± ceramic matrix composites support the available model of reaction. An alternative reaction mechanism is suggested whereby the dissolved and non-bridging oxygen atoms of the matrix di use and oxidize SiC and SiO x C y in the ® bre. The reaction yields carbon and silicon oxycarbide in the ® bre and SiO 2 which dissolves into the matrix. When the oxygen in excess in the matrix is consumed, the reaction stops and the phases undergo a reorganization in the reaction layer which generates two interphases, one carbon rich and the other silicon oxycarbide rich. These interphases are observed at the ® bre periphery in all glass or glass± ceramic matrix composites. § 1. Introduction Glass or glass± ceramic matrix composites are developed for thermostructural applications such as in aircraft parts submitted to severe thermomechanical stresses. Glass± ceramic is reinforced by means of long ® bres that are essentially stronger and less brittle than the matrix. This reinforcement yields composites with high strength and a`pseudoplastic' strain su cient to alleviate the catastrophic rupture mode of a brittle material. Amongst commercial ® bres, the SiC Nicalon 202 ® bre has been successfully used in the last years. The mechanical properties of Nicalon± glass matrix composites depend on the nature of the interphases that develop during material processing (Brennan 1988, Lancin 1991. A good characterization of the reaction between the ® bre and the matrix together with the understanding of this reaction are essential in order to elaborate composites with required speci® cations.Studies of the interphases produced in the course of the ® bre± matrix reaction have been conducted in a number of composites based on various matrices (Brennan 1988, Chen et al. 1989, Bonney and Cooper 1990. The most currently used are Li 2 O-Al 2 O 3 -SiO 2 (LAS) or CaO± Al 2 O 3 -SiO 2 (CAS) containing some additives such as niobium, arsenic or yttrium. In order to explain the formation of interphases, one usually refers to the model proposed by Cooper and Chyung (1987) which