The amount of nucleation on PETEOS was experimentally measured to quantify and understand the origins of selectivity loss during selective WCVD by the SiH4 reduction of WF6. Similar to our earlier findings for the H-2 system, nucleation is highest near exposed metal areas and displays a nonmonotonic dependence on the gas velocity. However, the dependence on the size of the exposed metal area proved to be considerably smaller. Reconciliation with a mathematical model indicates that selectivity loss is caused by an intermediate generated both on the deposition surface and in the gas phase. Nucleation and deposition rates increase with increasing temperature and silane concentration, and decreasing WF6 concentration. These comparable dependencies suggest a close relationship between the two processes.