Results of a study of laser-induced synthesis of ZnSe from a stack of elemental layers are presented. The process is traced by real-time, simultaneous measurement of both reflectivity and transmission of the irradiated thin films. Numerical solution of the thermal conductivity equation reveals that synthesis may develop at and above a well-defined threshold temperature, which is set by the melting of selenium. Further, it is shown that the whole process comprises three independent successive steps: interlayer mixing (or alloying), chemical reaction (i.e., formation of an amorphous compound), and crystallization of that compound. Among other findings, this analysis shows that large atom transport velocities (in mm/s) are necessary to account for the extremely short duration of the mixing step.