The undesired deposition of W on SiO2 during W chemical vapor deposition on Si with the WF6/SiH4 process has been studied by in situ time resolved mass spectrometry. This study shows that for [SiH4]/[WF6]<0.3 loss of selectivity occurs due to an autocatalytic reaction on the SiO2 surface. Based on the observations, a model for the process has been proposed. In this model, the rate limiting step is the formation of Si2H2n from two SiH4 molecules. This reaction requires a nucleation site, probably an impurity in the SiO2 or adjacent metallic areas, and is retarded by WF6. Subsequently, the Si2H2n reacts with WF6 to a W-containing species that will then also act as a nucleation site. When the density of W-containing species approaches monolayer coverage, these species react together with Si2H2n, thus forming areas of metallic W.