We present a suite of 36 high-temperature (900–1100 °C) experiments performed on 10 × 10 mm unjacketed cores of rhyolitic obsidian from Hrafntinnuhryggur, Krafla, Iceland, under atmospheric pressure. The obsidian is bubble- and crystal-free with an H2O content of 0.11(4) wt%. The obsidian cores were heated above the glass transition temperature (Tg), held for 0.25–24 h, then quenched. During each experiment the volume of the samples increased as H2O vapor-filled bubbles nucleated and expanded. Uniquely, the bubbles did not nucleate on the surface of the core, nor escape, conserving mass during all experiments. Within each isothermal experimental suite, the cores increased in volume with time until they reached a maximum, after which continued heating caused no change in volume (measured by He-pycnometry). We interpret these T-t conditions as representing thermochemical equilibrium between the melt and exsolved vapor. These experiments are modeled to recover the 1-atm, temperature-dependent solubility of water in the rhyolite melt. Our results define the magnitude of retrograde solubility (−7.1 × 10−3 wt% H2O per 100 °C) and provide estimates of the enthalpy and entropy of the H2O exsolution reaction [ΔH° = 17.8 kJ/mol, ΔS° = 107 J/(K·mol)]. We conclude by modeling the implications of retrograde solubility for the glass transition temperatures (Tg) of cooling volcanic systems at pressures relevant to volcanic conduits and the Earth’s surface. All volcanic systems cool; the effects of retrograde solubility are to allow melts to rehydrate by H2O dissolution as they cool isobarically, thereby depressing Tg and expanding the melt window. Ultimately, the melt is quenched at higher H2O contents and lower temperatures where the isobaric retrograde solubility curve “catches” the evolving Tg.