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A specific test procedure has been developed to compare the high temperature lifetimes of SiC/SiC microcomposites with various interphases in air and under mechanical loading. The interphases, namely pure pyrocarbon (PyC) or C(B) materials with uniform or variable boron contents in the thickness, were prepared by chemical vapour deposition (CVD). Uniform addition of boron in PyC interphases improved their oxidation resistance and consequently the lifetimes of the microcomposites. However, room temperature tensile tests have shown that this improvement occurs to the detriment of the mechanical properties even when a non-brittle behaviour is maintained. In the case of variable boron contents, compositional gradient interphases (CGI) in which boron content increases from the fibre interface to the matrix interface allow the mechanical fuse properties of PyC to be combined with the oxidation resistance of a C(B) material.