The vaporization of strontium, barium, and lanthanum from mixtures of their oxides with urania, zir-conia, and concrete is determined with the objective of understanding the release of these refractory fission products during the core/concrete interaction phase of a degraded core accident. The vaporization of uranium and the total mass vaporized are also determined. Three different concretes having silica contents ranging from 7 to 69 wt% are used to reflect the known range of reactor basemat compositions. In the experiments, the mixtures are vaporized at 2150 or 2400 K into flowing H2 or He-6 H2 gas. The total mass of material that was vaporized is determined by weighing the condensates; the masses of individual elements are determined by chemical analyses of the condensates. The phases present in the heated mixtures are inferred from electron probe microanalyses and X-ray diffraction analyses. Equilibrium calculations are performed using SOLGASMIX and a thermodynamic data base containing 112 gaseous and 108 condensed species. The partial molar free energy of oxygen is calculated from the equilibrium oxygen pressure established in the high-temperature reaction zone between the gas and the sample. Using this experimental data, the release to be expected in the molten core/concrete interaction phase of a severe nuclear reactor accident is estimated. The estimated release of strontium, barium, lanthanum, and uranium is <1% with a basemat concrete of low silica content (7 wt%) and decreases to <0.01% with a basemat concrete of high silica content (69 wt%). The estimated total mass release is ∼0.5% with all three concrete types.