Adsorption of tungsten hexafluoride on hydrogen-terminated silicon and silicon dioxide surfaces was investigated by in situ x-ray photoelectron spectroscopy in order to understand the selectivity loss occurring for chemical vapor deposition (CVD) of W films. It is found that WF6 molecules adsorb onto hydrogen-terminated silicon and silicon dioxide surfaces at faces at room temperature. When the substrates exposed to WF6 are annealed at 400 degrees C in ultrahigh vacuum, W and WF remain on silicon substrates, but all absorbates desorb from silicon dioxide surfaces. Only the reaction between WF6 and the substrates leads to the growth of tungsten films below 400 degrees C. If the WF6 gas is introduced with remote plasma-excited iir gas. induced WFx (x=0, 1,.., 6) molecules adsorb onto the silicon dioxide surfaces at room temperature. WFy(y=0,.., 3) adsorbates remain after the subsequent annealing at 400 degrees C, and cause the nucleation of W by CVD. Moreover, orine adsorbates are mainly bound to oxygen atoms after the annealing, implying the breaking of the Si-O bonds. The chemical pretreatment by an KF solution also breaks the Si-O bonds of the silicon dioxide surface, and creates a situation similar to that for the plasma-treated sample. The breaking of Si-O bonds additionally may contribute to the adsorption of WFx molecules.