The reacton of hydrogen with hydroxyl‐free vitreous silica has been measured for temperatures between 500 and 850 °C, pressures between 0.132 and 5.78 atm, and sample thicknesses between 0.057 and 1.06 cm. The results can be explained by a simple model often used to explain tarnishing reactions. This model assumes that the reaction rate is very fast compared to diffusion, and that a fixed concentration of trapping sites exists in the specimen.