To clarify the effect of halogen (X) upon the production of ion (M+) from alkali halide (MX) on a tungsten surface heated in vacuum, the ionization efficiency (beta (+)) of MX was measured as a function of surface temperature (T), and the effective work function (phi (+)) of the surface was determined, thus yielding the conclusions: (1) above similar to 1950 K, the surface was kept virtually clean at phi (+) = 5.2 eV; (2) as T decreased from similar to 1950 to 1400 K, beta (+) and phi (+) were increased to unity and above similar to6 eV, respectively, mainly by adsorption of neither MX nor X but of residual gases (RG) and (3) below similar to 1300 K, each of beta (+) and phi (+) showed different patterns among MX's because of coadsorption of MX and RG. The work function (phi (e)) for thermal electron emission exhibited the same temperature dependence as that of phi (+) and phi (+) - phi (e) was constant at similar to0.7 eV above similar to 1600 K, supporting the above conclusions.