Nicalon SiC fibres were heat-treated in various atmospheres and at various pressures. Initially CO, nitrogen and air were used as the heat-treatment environment at one atmosphere pressure. Microstructural changes and any related strength degradation or improvement were measured for the heat-treated fibres. After heat-treatment in the temperature range 1000 degreesC-1600 degreesC, each sample showed different weight changes. Thus, in air, a weight gain was observed with increasing temperature, whereas in CO and N-2, weight losses were observed but with a smaller weight loss observed for CO. Moreover, carbon monoxide had a significant effect on the strength retention of the fibres. Since the lowest weight loss was observed after heat-treatment in CO at one atmosphere, high pressure CO gas was used to heat-treat Nicalon fibres between 1000 degreesC and 1700 degreesC and the resulting fibres were analysed by single-filament strength testing, scanning electron microscopy, and X-ray diffraction. The results were completely different compared with those in one atmosphere of CO. As the temperature increased, weight and strength increased whereas at one atmosphere pressure, both weight and strength had decreased. The weight increase was because of surface reaction between the CO atmosphere and the SiC fibre and/or because of deposition of carbon from the pressurised CO gas, giving the fibre a surface carbon coating. Carbon coating of a fibre is a beneficial property for CMCs since it provides a weak interface which facilitates pull-out during fracture.