This document provides requirements for the selection of safe combinations of metallic cylinder and valve materials and cylinder gas content. The compatibility data given is related to single gases and to gas mixtures. Seamless metallic, welded metallic and composite gas cylinders and their valves, used to contain compressed, liquefied and dissolved gases are considered. NOTE In this document the term "cylinder" refers to transportable pressure receptacles, which also include tubes and pressure drums. Aspects such as the quality of delivered gas product are not considered.