The nucleation of tungsten on bare Si and SiO2/Si surfaces by Ar ion laser induced CVD has been studied with high speed real-time specular reflection and scattering. Our experimental results show that the induction time of tungsten nucleation decreases with laser power and is longer on thicker SiO2 surfaces than on thin native SiO2 surfaces at the same laser power. The activation energy of tungsten nucleation on native SiO2/Si obtained from an Arrhenius plot of induction time is about 33 kJ/mol at temperatures less than 1000 K and 16 kJ/mol at temperatures higher than 1000 K. The results on bare Si with and without H2 reveal that the initial reaction mechanism appears to be the same in both cases. An Arrhenius plot of induction time shows two different apparent activation energies at different temperature ranges, which suggest that W nucleation is controlled by H atom desorption from the Si surface at temperatures less than 714 K and is dominated by Si atom reduction of WF6 molecules at surface temperatures greater than 800 K.