The room-temperature tensile properties and constituent microstructures of a unidirectionally reinforced SiC/RBSN composite have been investigated after heat treatments at 1400, 1600, or 1800°C in nitrogen for up to 100 h. The composite consisted of ∼ 24 vol% of aligned 140 μm diameter, continuous length, chemically vapour deposited SiC fibres in a ∼ 40% porous silicon nitride matrix. The composites heat treated at 1400°C for up to 100 h showed elastic modulus, first matrix cracking strength and ultimate tensile strength values similar to those of the as-fabricated composites, but those heat treated for 1 h beyond this temperature displayed losses in all three properties. Recrystallization of the SiC fibres, loss of carbon from the carbon-rich interface coating on the fibres, and dissociation of the RBSN matrix are the reasons for the loss of mechanical properties.