A negative velocity gradient near the inner-outer core boundary might be adequately explained by the existence of heat-producing elements in the region. Geochemically, there should be about one-third to more than a half of the earth's total resources of heat producing elements present below the upper mantle, which is also consistent with observations of heat flux at the earth's surface. The results of high-pressure experiments further suggests that the heat-producing elements would exist as dioxides, which are chemically incompatible with the major mantle silicates, under the temperature and pressure conditions prevailing in the lower mantle. The great density of these heat-producing dioxides (probably greater than the liquid iron-nickel core) may cause them to be transported to the inner-outer core boundary (the F zone).