Secondary ion mass spectrometry (SIMS) and energy dispersive X-ray spectroscopy (EDX) were used to analyse interphases which develop at the fibre-matrix interface during fabrication of SiC nicalon fibre-LAS (Li2O, Al2O3, SiO2) or LAS + Nb2O5 glass matrix composites. SIMS was performed on fibres extracted from composites by dissolution of the matrix in hydrofluoric acid (HF). The composition of the interphases which remain on the fibre periphery was studied using sputter-depth profiling. EDX analysis of the interphases was performed on nicalon-LAS + Nb2O5 composite cross-sections. These investigations show that in both composites the reaction products at the fibre-matrix interface consist of a carbon layer and a. silicate phase, rich in oxygen, located between the carbon layer and the fibre core. The analyses also demonstrate the efficiency of SIMS for analysing compositional changes in the near surface of fibres of small diameter.