A major challenge to using proppant in geothermal reservoirs is the possible reaction of proppants with reservoir rock and hydrothermal brines at high temperature, which could compromise their mechanical integrity, leading to increased fracturing and decreased efficacy. We reacted proppant materials of several compositions with hydrothermal brine and reservoir rock powders, then performed crush tests to assess how chemical reactions modify mechanical characteristics of the proppants. One set of samples was reacted for 7 days at 250°C and 15.9 MPa and another for 63 days at 200°C and 13.8 MPa. We find that reactions between proppant materials, brines and reservoir rock lead to minor etching of proppant surfaces and precipitation of secondary minerals, but proppants maintain mechanical integrity. The results of this study indicate that a wide range of proppant materials may be suitable for use in geothermal applications without the risk of mechanical degradation due to hydrothermal alteration.