Undoped, thin silicon films have been deposited at different temperatures on fused quartz substrates by low‐pressure chemical vapor deposition. The heating of these films by continuous wave, argon‐ion laser beam irradiation has been studied.Insitu, normal incidence reflectivity measurements have been obtained at specified locations in the semiconductor films. Melting and recrystallization phenomena have been probed by transient measurements. The static film reflectivity at elevated temperatures, up to about 1400 K, has also been measured. The temperature field has been analyzed numerically, using a modified enthalpy model. Thin‐film optics were used to calculate the argon‐ion laser light absorption in the polysilicon layer and the transient reflectivity response to the probing laser light. The predicted and experimentally measured reflectivity histories have been compared. The initial stages of the phase change process have been captured by high‐speed photography.