We have studied laser induced crystallization and amorphization reactions in tellurium and tellurium alloy thin films, which are of interest for use in optical data storage. The writing characteristics of these films are determined by plotting changes in film reflectivity against the parameters of laser power and pulse width. The laser pulse conditions necessary for film melting, crystallization, and amorphization are identified and then checked against the results of a numerical heat diffusion model. In this way we determine the critical quenching rate for formation of the amorphous phase in pure tellurium (2 × 109 C/sec). Minimum crystallization times are determined and found to increase by orders of magnitude with addition of alloying elements such as germanium or selenium. TEM analysis reveals significant differences in the crystallization mechanisms of different alloy systems.