The thermal outgassing and helium permeation characteristics of Code 9658 machinable glass–ceramic are discussed. Up to 600° C, it is shown that this material can be treated like glass vacuum system components. The quantities of outgassing constituents under vacuum conditions are given and the need for vacuum baking freshly machined material is demonstrated. Although the material can be used up to 1000° C, between 600° and 1000° C some fluorine evolution from the material takes place and could present a corrosion problem to some systems. Helium permeability of this material is an order of magnitude lower than for Code 7740 glass.