This study is focused on sorption and determination of distribution ratios (R d) of cesium and selenium on mudrock, which is the potential host rock for waste disposal in Taiwan. Batch tests including sorption kinetic and isotherms tests have been performed in synthetic groundwater at aerobic and anaerobic conditions which might be found in the deep geologic environment. It is found that R d for sorption of cesium did not have an obvious difference in both conditions with various contact time. However, R d in anaerobic condition for sorption of selenium was greater than that in aerobic condition. Selenium is a redox sensitive element and its solubility in reducing conditions is controlled by the formation of metallic or precipitable selenium. It demonstrated variation of R d with time in both conditions for Se sorption kinetic experiments was equal (10 ml/g) and indicated a part formation (10%) of precipitable selenium (Se0, FeSe or FeSe2) in the solution. Moreover, it was not enough to form precipitable selenium completely in reducing condition as to insufficient experimental period (2 weeks) and in the presence of Fe2+. The Freundlich and Langmuir isotherm for the concentration ranges (i.e., 10−3–10−7M) conducted in both conditions seem to be adequate to quantitatively describe the sorption of cesium and selenium, respectively.