The thermal expansivities (&agr;) of MgO and high-pressure phases of CaO, CaMgSi2O6, and Fe at ultrahigh pressure are obtained by comparing existing shock compression and temperature measurements to 300 K compression curves constructed from ultrasonic elasticity and static compression data. For MgO, &agr; can be represented by: &agr;=&rgr;0&ggr;0CV(&rgr;0/&rgr;)0.5¿0.5/KT, where &ggr; is the Gr¿neisen parameter, CV is the constant volume specific heat, KT is the isothermal bulk modulus, and &rgr; is the density. Using this expression, the thermal expansivity of MgO is 28--32¿10-6 K-1 at the pressure of the top of the lower mantle and 10--16¿10-6 K-1 at its base (at 2000 K). New data for &agr; of &egr;-Fe, together with an inner core temperature of 6750 K, constrain the density of the inner core to be 5¿2% less than the density of &egr;-Fe, implying the inner core contains a light element. ¿ American Geophysical Union 1993