Abstract. Optical recording characteristics of thin films of Set 100-xGex (where x is the weight percentage) are studied using an Ar+ laser at A = 488 nm. The variation of optical transmittance due to the laser illumination and the corresponding crystallization of the thin films are examined to provide an understanding of the variation of the microstructure in the thin films during the optical recording process. The laser-induced crystallization is associated with the growth of hexagonal selenium along one or several orientations. The diffraction efficiencies of real-time holographic gratings are measured during the writing process.