Explosive crystallization of Cu implanted amorphous silicon during irradiation by a 32‐ns FWHM ruby laser pulse has been studied using time‐resolved reflectivity measurements and Rutherford backscattering spectrometry. From interferences in the reflectivity, the position and the velocity of the self‐propagating melt have been deduced as a function of time. A maximum average velocity of 13±2 m/s has been obtained. The reflectivity behavior indicates the presence of crystalline nuclei in the melt.