Abstract The hydrothermal degradation of alumina-based proppants, which can lead to significant loss of fracture conductivity, has been the topic of recent papers. Most of these studies were necessarily compromised by being performed on either too short of a time scale to produce geochemical effects or by using unrealistic temperature conditions to accelerate the geochemical reactions. Those studies, however, provided insight that geochemical reactions likely occur during production from hydraulically generated fractures and gave evidence of possible long-term proppant instability. This paper presents proppant-pack permeability and proppant crush-strength data collected from a selection of hydrothermal tests performed in sealed test cells packed with proppant and formation material with no flow and no mechanical closure stress. Long-term proppant performance was determined by evaluating the proppant after 0, 25, 45, 60, 90, and 180 days of hydrothermal exposure at 300 and 450°F, resulting in verification that proppants continuously degrade with time at all temperatures. A typical long-term proppant-testing result was 80% loss in proppant-pack permeability and 40% loss in proppant crush strength with just 180 days exposure at 300°F at typical reservoir pH. These tests were performed both with and without formation material present to demonstrate the impact of reservoir/proppant compatibility. Scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDX) analyses verified that dynamic molecular rearrangement occurs over the entire temperature range; albeit, at an accelerated rate as temperature increased. Extensive effort to quantify both the accuracy and repeatability of testing procedures used in this study were made and will be presented. This involves measurement of pack permeability using water before and after hydrothermal exposure, and single- proppant-grain crush-strength determination using Weibull statistical analyses. This paper presents substantial data to support the importance of proppant compatibility when selecting the best proppant for long-term fracture conductivity. And it suggests that methods in addition to API crush and conductivity procedures need to be developed and implemented to properly rank proppant performance.