Transmission electron microscopy in conjunction with x-ray photoelectron spectroscopy (XPS) was used to study the possible microstructural changes occurring in electron-beam-evaporated arsenic trisulfide thin films, when deposited on a sodium chloride substrate from room temperature to 250 °C. These films were found to be amorphous up to 150 °C, while a transition from the amorphous to the polycrystalline phase was observed to take place at 200 °C. The electron-diffraction results showed that the crystallized sulfur-rich phase was predominantly As2S5, along with As2S3. The XPS spectrum showed shifts in binding energy values for As towards higher energies. A shift of 0.9 eV for the As (3d) spectrum was attributed to the transition from the As2S3 to the As2S5 phase occurring at ∼200 °C.