The effective work function (φ +) for producing a positive ion on a polycrystalline rhenium filament heated at a residual gas pressure of 2×10 −7 torr was previously found to increase by up to ≈1.5 eV at surface temperatures of ≈1000–2200 K. In order to analyze the causes of this increase, the emission current of Li + produced from LiI was measured as a function of both the elapsed time after flashing the filament with the sample beam shutter closed and the delay time before opening the shutter. When the sample beam flux ( N) was as low as 6.6×10 12 molecules cm −2 s −1, the increase was caused by adsorption of residual gas molecules (RGM). When N was increased ten times at as low a temperature as 1160 K, on the other hand, the increase due to the adsorption was suppressed by incidence of LiI, and hence θ + was a complex function of coadsorption of RGM and LiI.