The basal vitrophyre of the Topopah Spring Member of the Paintbrush Tuff at Yucca Mountain, Nevada, is a perlitic glass with 2.8 to 4.6% water. The dehydration of this vitrophyre was studied by thermogravimetric analysis (TGA) and by isothermal heating at 50¿, 100¿, 200¿, and 400 ¿C for 3.4 years followed by 1.1 years of rehydration at high controlled humidity (~79% relative humidity). No crystallization of the glass was observed in long-term dehydration or rehydration; the only observed chemical alteration was loss of up to 60% of original fluorine. TGA studies show a characteristic two-stage dehydration of the vitrophyre, with two-thirds to three-fourths weight loss occurring most rapidly at temperatures ranging from 278¿ to 346 ¿C in 10 ¿C/min heating experiments. The remaining water, about 1% in all of the vitrophyre samples studied regardless of total water content, is lost only on second-stage heating to temperatures above 650 ¿C. Long-term isothermal heating at ≤400 ¿C releases only the first-stage water. Loss of essentially all first-stage water occurred in less than 1 hour at 400 ¿C; proportionately lower losses were obtained at 200¿ and 100 ¿C. Small (0.2%) water loss occurred in the 50 ¿C experiment. A time-temperature-dehydration diagram generated from the isothermal heating data shows a clustering of dehydration contours that are the equilibrium equivalent of the rapid first-stage water loss in dynamic TGA experiments. These dry-heating experiments provide an end-member characterization of glass transformations for comparison with water-saturated heating experiments in which glass alteration is prominent. ¿ American Geophysical Union 1993