Improvement of the reactor water cleanup system is one of the most effective methods for radiation reduction in boiling water reactor plants. This necessitates development of a heat-resistant adsorbent for direct use to remove cobalt under reactor conditions (285°C, 70 kg/cm2). We have developed a suitable adsorbent by impregnating granular sponge titanium with titanium oxide. Adsorption of Co2+ on titanium oxide was evaluated as a function of temperature. Selectivity experiments showed that corrosion products, such as Co2+, Ni2+, and Cu2+, were preferentially adsorbed at high temperatures. The data suggested that chemical adsorption occurred with the formation of insoluble cobalt metatitanate on the TiO2 surface. In-plant tests, carried out to evaluate the performance using actual reactor water, verified the applicability of the new adsorbent to the high-temperature reactor water cleanup system.