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We determined near-infrared (NIR) molar absorptivities of hydroxyl group (4500 cm⁻¹) and molecular water (5230 cm⁻¹) in rhyolitic glasses at room temperature and at high temperature, 400–600 °C. Five rhyolitic glasses with 0.5–2.8 wt% total water were analyzed by FT-IR microspectroscopy at room temperature and at 50 °C intervals in the range of 400 to 600 °C using a heating stage. The total water contents in the rhyolitic glasses were measured by Karl-Fischer titration. Based on these data, the linear molar absorptivities of the 4500 and 5230 cm⁻¹ bands at room temperature were determined to be 1.42 ± 0.12 and 1.75 ± 0.08 L/mol·cm, respectively. The integral molar absorptivities obtained are 285 ± 24 and 239 ± 11 L/mol·cm² for the 4500 and 5230 cm⁻¹ bands, respectively. These values can be used to determine the concentrations of hydroxyl group and molecular water in rhyolitic glass with 0.5–2.8 …