Uniform or composition-graded C(B) (i.e., boron-containing carbon) interphases in SiC/SiC model microcomposites were characterized by transmission electron microscopy after tensile tests and thermal ageing in air. A specific method was developed to prepare thin longitudinal sections of the tested specimens. Deflection of matrix cracks occurs within the uniform C(B) interphase, as long as its anisotropy remains high enough (i.e., when the boron content is not too high). It takes place close to the most anisotropic layer (i.e. that containing 8 at% of boron) in composition-graded interphases. In both cases, the crack deflection path does not reach the fibre, a feature which is consistent with the good mechanical properties. After ageing in air under tensile loading beyond the proportional limit (600 degrees C; sigma = 800 MPa), the composition-graded interphase (made of five sublayers in which the boron content increases from 0 at% near the fibre to 33 at% near the matrix) was observed to act as a glass-forming protection, the pyrocarbon sublayer (at the fibre surface) remaining unoxidized.