SummaryTwo experimental SiC fibre‐reinforced yttrium magnesium aluminosilicate (SiCf‐YMAS)‐type ceramic‐matrix composite (CMC) materials fabricated (i) by the glass process and (ii) by chemical precursor infiltration have been studied by light microscopy, transmission electron microscopy (TEM), high‐resolution electron microscopy (HREM) and energy‐dispersive X‐ray spectroscopy (EDS). The distribution of the fibres inside the composite as well as the average diameter of fibres have been determined by image analysis. The microstructure of the YMAS matrices has been characterized by TEM observations. YMAS matrices are formed of two main phases, cordierite and β‐yttrium silicate (Y2Si2O7). Two minor phases (mullite and spinel) have been found to crystallize inside the cordierite and the yttrium silicate crystals. Fibre‐matrix interfaces have been observed in HREM. A thin turbostratic carbon layer (20–30 nm) has been imaged in both composites at the fibre‐matrix interface. It crystallizes along the matrix interface and grows inside the fibre, forming a diffuse interphase. The carbon layer is believed to be the consequence of reaction between oxygen in the matrix and SiC nanocrystals of the Nicalon fibres.