A composite reverse buckling disc assembly consisting of two portions, a concav-convex dome portion (2) and a support portion (6). The support portion (6) includes an annular outer part (8) for clamping between opposed inlet and outlet support means (12, 14) of a safety pressure relief device and an inner part (10) for location in the bore through said device. The dome portion (2) is located on the support portion (6) such that, on reversal of said dome portion (2) when it is subjected to the critical pressure of the assembly, said reversing dome portion (2) engages with the inner part (10) of the support portion (6) with sufficient force to break said inner part (10) away from the clamped outer part (8), whereby the dome portion (2) is released from the assembly and full-bore discharge through the safety pressure relief device occurs.