Abstract: The performance and construction of a combined EI/CI ion source designedfor GC/MS are described. The pressure of reagent gas in CI chamber is about1 Torr while the pressure in the remainder of the source housing does notexceed 7×10~(-5) Torr. In the electron bombardment mode of operation, thesource is almost exactly as sensitive as the original, in that it gives thesame ion current for a standard does of air