Our previous studies found evidence for a selectivity loss mechanism that is initiated by desorption of tungsten subfluorides formed by the reaction of WF6 with metallic tungsten surfaces. When a hot (≂600 °C) tungsten surface is exposed to WF6, a nonvolatile subfluoride, apparently WF4 , will accumulate on a nearby SiO2 surface held at ≂30 °C. Disproportionation of this tungsten subfluoride upon heating above 350 °C produces a reactive state of tungsten that induces rapid selectivity loss when exposed to chemical vapor deposition conditions. The net effect of this tungsten subfluoride desorption–disproportionation mechanism is the transport of tungsten from a tungsten surface to the silicon dioxide surface. Isolation of a higher tungsten subfluoride, apparently WF5, has been accomplished by cooling the SiO2 surface to ≂−45 °C under otherwise similar experimental conditions. Preliminary Auger spectroscopic results yield a stochiometry of F/W=4.9±0.5 for this subfluoride, and temperature programmed desorption results are consistent with the behavior of tungsten pentafluoride. Upon heating WF5 multilayers, most of the compound desorbs, but some reaction and disproportionation occurs leaving a reduced state of tungsten on the SiO2 surface. It seems likely that tungsten pentafluoride is the key volatile species in a tungsten transport mechanism that can initiate selectivity loss.