The formation of W through the reduction of WF6 by Si is monitored in situ using a wavelength adjustable reflectometer.The reflectance-time relation can be understood and modeled by assuming island growth and a statistical distributionof the island thickness. The model is supported by SEM and Auger observations. The effect of surface layers like nativeoxides or a plasma treatment on the inhomogeneous Si consumption by the reaction between Si and WF6 (gouging) and itseffect on the reflectance-time relation are understood. The model is also applicable in the case of renucleation during theH2 reduction of WF6. A renucleation step consists of the deposition of Si from SiH4 followed by the Si consumption byWF6. A renucleation step reduces the surface roughing which occurs during the H2 reduction process.