This International Standard 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, and determination of the type and arrangement of the constituent glass layers. Two burning temperatures are given, related to the stability of the glass reinforcement at elevated temperatures. Application to other composite components should be considered in the referring standard.