This document specifies a method for the determination of constituent materials of a test sample cut from a glass-reinforced plastics (GRP) component intended for use in a piping system. It includes determination of resin, glass, aggregate and filler contents. It is also applicable to the determination of the type and arrangement of the reinforcements. If used to determine the amounts of constituent materials in layered constructions it may be necessary to separate the laminate layers by cutting or splitting and testing each separately.