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Thermal properties of chalcogenide AsxS100−x glasses in the glass transition region have been studied by modulated-temperature differential scanning calorimetry (MTDSC). All samples in this work were given the same thermal history by heating to a temperature above the glass transition, equilibrating and then cooling at a rate of 5°C/min to a temperature of 20°C. The reversing and non-reversing heat flows through the glass transformation region during both heating and cooling schedules were measured and the values of the parameters Tg, ΔH, Cp and ΔCp, which characterize the thermal events in the glass transition region, were determined as a function of the glass composition. The structurally determined parameters Tg, ΔH, Cp and ΔCp reveal major extrema when the composition of AsxS100−x glass becomes As40S60, that is the same as the composition of the corresponding stoichiometric compound. In addition, we observe “small thresholds” in these properties at 28.5 at % As (As28.5S71.5) around the same composition as that reported in the As-Se glasses. No such thermal analysis had been done on the AsxS100−x glasses previously. It is shown that AsxS100−x glasses where x < 25 at % As are formed from two glass phases. From MTDSC measurements, it was possible to establish the probable composition of the high temperature glass phase and from Raman spectroscopy it was possible to correlate the MTDSC results with the structure of the As-S glasses.