Investigation is made of the desorption of He atoms from thin films. Three different experimental geometries are used to study the dependence on substrate material, film temperature, and duration of the heat pulse. The major conclusions are that (a) the speed distribution is similar to a Maxwellian, (b) the angular distribution is strongly focused about the normal direction, (c) desorption time constants differ systematically from estimates, and (d) a phonoatomic effect analogous to the photoelectric effect is observed. The measurements are analyzed in terms of several theoretical models. Conclusions are drawn about the time evolution of the film thickness and temperature.