As the world's demand for energy continues to grow, exploration and production activity increasingly involves operations in high-pressure, high-temperature (HPHT) conditions. HPHT domains include geothermal, steam-injection, and ultradeep wells. These environments introduce difficult technical challenges. A major concern is the set-cement integrity. To maintain zonal isolation throughout the life of an HPHT well, the cement sheath must perform reliably at temperatures that can exceed 315°C [600°F]. When exposed to temperatures higher than 110°C [230°F], set Portland cement undergoes strength retrogression unless additional silica is incorporated in the cement formulation. This phenomenon has been studied extensively. However, less attention has been paid to the effects of other additives on the stability of set Portland cement in an HPHT environment—weighting materials in particular. Weighting materials have largely been assumed to be inert with respect to set Portland cement; however, the present study reveals that this assumption may be false in HPHT environments in which well temperature exceeds 260°C [500°F]. Some weighting agents may react with the set Portland cement, causing strength loss and increased permeability. Fortunately, this effect can be prevented, and the set-cement integrity can be preserved.