Ultrasonic interferometry was used in combination with synchrotron X-radiation to determine the compressional and shear wave velocities and unit-cell volumes of ε-FeSi (cubic B20 structure) at room temperature and pressures up to ∼12 GPa. The data collected during compression are compared with those collected during decompression after heating to release stress within the sample cell. By fitting all of the decompression unit-cell volume and sound velocity data to third-order finite-strain equations, we obtain the adiabatic zero-pressure bulk and shear moduli and their first pressure derivatives: K_S0=169.3 (8) GPa, G₀=116.3 (4) GPa, K'_S0=6.5(3), G'₀=3.0(1). The bulk modulus obtained from this study is in good agreement with those of some previous experimental studies, but significantly lower than those obtained by first-principle calculations. This study presents the first direct measurement on the shear properties of this phase.