Changes in fibre-matrix interface during the heat treatment of Ti6Al4V matrix reinforced with carbon-coated SiC fibres, have been studied using XPS, AES and SEM/EDS (energy dispersion spectroscopy) techniques. The samples were characterized before and after heating at different temperatures up to 600 degrees C with long exposure times up to 1000 h. Particular attention was paid to the C/matrix interface, where high temperature treatment could lead to the formation of titanium carbide. Due to the experimental difficulties to separate the contributions of TiC and SiC, the interaction between titanium and carbon was simulated by heating Ti6Al4V and pure Ti foils covered with graphite. Obtained interfaces have been analysed by XPS depth profiling. Experimental results were supported by theoretical calculations of carbon diffusion.