The volume of ϵ-FeSi has been measured to pressures of 50 GPa. No high pressure transformations are observed in this system, including after laser-heating at high pressures. The bulk modulus of this phase is 209 (¿6) Gpa, with a pressure derivative of 3.5 (¿0.4). This high bulk modulus will slightly elevate the thermochemically-inferred pressure conditions at which chemical reactions between iron and mantle silicates should commence at mid-mantle depths. Moreover, because of the relatively low pressure derivatives of the bulk moduli of FeSi alloys manifested both in our data and in previous shock results, silicon as the sole light alloying component of the outer core is unlikely to produce a sufficiently large change in the bulk sound speed of the outer core relative to pure iron liquid. Therefore, silicon is not likely to be the primary alloying component in the outer core, unless its effect on the elasticity of the outer core is fortuitously offset by other light alloying constituent: its role is probably that of a subsidiary constituent. Thus, silicon is unique (to date) among proposed major outer core alloying constituents in being potentially precludable on purely geophysical grounds. ¿American Geophysical Union 1995