A horizontal hot-wall chemical vapor deposition (CVD) quartz reactor with rectangular cross section was used to study the effect of different process conditions on the nucleation of tungsten on SiO2 during selective WCVD by the H-2 reduction of WF6. The experimental procedure included placing a metallic surface at the center of the reactor, and small samples of SiO2 at different positions both upstream and downstream with respect to the metallic surface. Digitized scanning electron microscopy micrographs were used to determine the particle size distributions of nuclei on the SiO2 surfaces. We found that the amount of nucleation-on SiO2 decreases when smaller metallic surfaces are present and for lower temperatures and shorter process times. Although nucleation was always greatest on SiO2 samples closest to the metal sample, the effect of flow rate depended on the position of the SiO2. A statistical nearest neighbor analysis indicated a clustering of W nuclei on the SiO2. A simplified mathematical model was developed to predict concentration profiles of a gaseous intermediate generated at the metal surface during the thermal decomposition of the source gas. This intermediate has been proposed as being the reactive species that causes nucleation on SiO2 surface. Qualitative agreement between experimental and theoretical results reinforce the proposed role of the intermediate with this species being characterized by a short lifetime.