The microstructural stability of SCS-6 SiC monofilaments was determined by measuring the average grain size, tensile strength and critical length of the fibres as a function of various annealings in vacuum (approximate to 0.1 Pa) at different temperatures (1400-1600 degrees C). The average grain size, calculated from X-ray diffraction line broadening, increased from 23 nm for the as-received fibres to 46 nm for fibres annealed for 2 h at 1600 degrees C. The corresponding tensile strength measured at room temperature dropped from 3.6 GPa for the as-received fibres to approximate to 2 GPa for the treated fibres. Simultaneously, the average critical lengths, measured using the glass-slide technique decreased from 0.37 mm for the as-received fibres to an average of 0.23 mm for the heat-treated fibres. The degradation of the mechanical properties was attributed to a combination of coarsening of the beta-SiC grains as well as interactions with the annealing environment, namely the vacuum hot-press chamber.