Small amounts of silicon evaporated onto a clean tungsten tip in ultrahigh vacuum show two distinct types of surface diffusion. At about 550 °K boundary free diffusion occurs on the [111] zones. Above 950 °K diffusion occurs with a sharp boundary and an activation energy of 1.5 eV. The activation energy of thermal desorption of silicon from tungsten depends on the surface coverage and varies from 5.6 to 6.2 eV. Annealing the silicon covered tip at ≈ 1400 °K causes diffusion of silicon into the interior of tungsten and produces a different and stable pattern, probably that of a tungsten silicide with a work function of 4.8 eV.