A molecular beam of diatomic metal halide (MX, e.g., LiCl, NaBr, KF, RbBr, CsF, TlCl or InI) was directed onto a polycrystalline surface of refractory metal (e.g., W or Re) heated in high vacua, and the ion current of M+ emitted from the surface was measured as a function of either (1) surface temperature (T approximate to 800-2300 K), (2) the elapsed time (t approximate to 0-10(3) s) after having made the surface essentially clean, or (3) the introduced gas pressure (P approximate to 10(-5)-10(-3) Pa) of air or oxygen, while the sample beam flux (N) incident upon the surface was kept constant in the range of 10(12)-10(14) molecules cm(-2) s(-1). The data thus achieved were analyzed by our theory to determine the effective work function (phi(+)) for ion emission. In addition, the work function (phi(e)) for electron emission from each surface was measured under various conditions, thereby making it possible to determine the thermionic contrast (Delta phi* = phi(+) - phi(e)). The values of [phi(0)(+), phi(0)(e) and Delta phi(0)* in kJ mol(-1)] determined with an essentially clean surface of each metal heated to a high temperature (usually above ca. 1800 K) if, a readily attainable high vacuum (2 x 10(-5) Pa) are as follows; Nb [463 +/- 10, 388 +/- 5 and 75 +/- 10], Mo [480 +/- 6, 424 +/- 3 and 56 +/- 6], Ta [493 +/- 5, 413 +/- 5 and 80 +/- 5], W [504 +/- 5, 434 +/- 6 and 70 +/- 6], and Re [530 +/- 5, 476 +/- 5 and 54 +/- 5]. With decreasing temperature (usually from ca. 1800 to 1400 K), both phi(+) and phi(e) become higher by up to ca. 100 kJ mol(-1) than phi(0)(+) and phi(0)(e), respectively, mainly owing to the adsorption of residual gases (especially of oxygen). However, Delta phi* itself remained virtually constant at Delta phi(0)* with little dependence upon T, t, P and N in the above respective ranges.