Resonant ultrasound spectroscopy (RUS) is a powerful tool for determining the elastic properties of solids at high temperature. Before RUS can be used to measure the pressure derivatives of elasticity, however, effects of boundary conditions between the pressurizing gas and specimen must be understood. Data are presented that show effects of different pressurizing gases on the apparent or measured values of (∂G/∂P)T of fused silica spheres, where G is the shear modulus, P is pressure, and T is temperature. The value of (∂G/∂P)T is found to depend on the molecular mass, M, of the pressurizing gas via −3.425(0.032)−5.9(1.6)×10−3M, when M is in grams. Extrapolating to zero gas mass gives (∂G/∂P)T=−3.42(0.03), a value bracketed by results from previous plane-wave, ultrasonic measurements. An alternative analysis in which effects of the pressurizing gas are removed by theoretical concerns is also presented and suggests that (∂G/∂P)T is −3.32(0.03). Our results quantify the effect of pressurizing gas on measured pressure derivatives of G when using RUS and indicate that systematic problems in transducer-specimen bonding from previous acoustic plane-wave measurements on fused silica at elevated pressure are not yet measurable.