Mark formation by laser writing in an overcoated thin‐film tellurium alloy‐based optical storage media with a trilayer design was studied by transmission electron microscopy, scanning electron microscopy, optical microscopy, differential interference contrast optical microscopy, and optical interferometry. Mark formation was observed to occur through coalescence of the relatively uniform amorphous tellurium alloy film into discrete particles. The mark microstructure and, correspondingly, the optical properties of the mark, were dependent on the write laser power. At low power, incomplete breakup of the film occurred. At high power, the film coalesced into spherical particles. Laser writing also caused changes in the organic overcoat layer.