Selenium (Se) is one of the elements that are included in environmental quality standards for soils because it is highly toxic to humans. Although natural Japanese soils do not have a high Se content, Se contamination of soils will become a big problem because its consumption for industrial purposes is increasing. From the viewpoint of radioactive waste management, the fate of the element in a soil environment is also interesting because the long-lived fission product Se-79 (half-life: 1.1 million y) is an important radionuclide for long-term dose assessment. Soil organic substances influence pollutant sorption on soil. Recently, many studies have shown that humic acid (HA), a soil organic substance, adsorbs some radioactive elements, such as U and Am. However, no study has been carried out for Se adsorbability onto HA or for the sorption kinetics. Thus, our aim in this study was to clarify the sorption behavior of Se on HA. We investigated influences of solid-liquid ratios and Se concentrations by measuring the amount of Se sorbed on HA and the time to reach the sorption equilibrium. We also determined size distributions of HA-sorbed-Se under increasing HA amounts and co-existing Se amounts using an ultrafiltration technique. Finally, we looked at a suitable determination method for Se concentration in the liquid phase to obtain an isotherm equation. Humic acid was purchased from Wako Pure Chemicals Industries Ltd. Batch sorption tests using Se-75 (half-life: 119.8d) were carried out at elevated amounts of HAs, that is, 100, 200, 500, and 1000 mg. Each HA sample was placed in a plastic bottle (30 mL), and then 9 mL of deionized water (Milli-Q quality) were added. One mL of Se solution, which included 8 kBq of Se-75 with 0.1-3 micro-g of stable Se as a carrier, was added to each sample bottle; the initial concentrations of Se in the bottles were adjusted from 10 to 300 micro-g/L. Then, the suspension was shaken using an end-over-end shaker. In order to determine the time required to reach the sorption equilibrium, the sample suspensions were destructively sampled at times from three hours to twenty days after the addition of Se-75 to them. Each sample suspension was then filtered through a 0.45-micro-m membrane filter. Gamma rays of Se-75 in the filtrate were measured with an NaI scintillation counter (Aloka ARC-380). In addition, at the time of equilibrium, each 0.45-micro-m sample suspension was sequentially ultrafiltered with cutoff molecular weight filters of 10k, 5k, and 3k Daltons (Da) to get the molecular size distributions of the Se-sorbed HA. All filtrates were measured with an NaI scintillation counter. 　The results were as follows. 1) The time required for the sorption equilibrium depended on the solid-liquid ratios and the Se concentrations. 2) The amount of Se sorbed in each molecular size range decreased in the order 3kDa> > 10kDa-0.45micro-m > 5KDa-10kDa > 3kDa-5kDa. 3) A linear relationship was observed between the logarithm of the concentrations of HA-adsorbed Se and the logarithm of the concentration of Se in the liquid phase, under the equilibrium condition. Therefore, Se sorption onto humic acid followed the Freundlich isotherm, which is described by S = K  Ce•EXP(n), where S is the amount of Se sorbed on the solid (micro-g/g solid), Ce is the equilibrium concentration of Se in solution (micro-g/mL), and K and n are Freundlich constants. When the liquid phase was assumed as the filtrate which passed through the 0.45 micro-m filter, K, n and correlation coefficient (R) were calculated as 216, 0.80 and 0.89 respectively, using all the data from the Freundlich isotherm. Since K and n depended on solid-liquid ratios, low R was observed. If only the data of one solid-liquid ratio condition were applied to the Freundlich isotherm, R had a high value, i.e. 0.99. Additionally, when the 3kDa filtrates were regarded as liquid phase, K, n, and R were 367, 0.82, and 0.99, respectively. In both cases, n values were lower than 1; n < 1 means a limited number of specific high-affinity sorption sites are available for the sorbate molecules. From the results, we concluded that the use of the <3kDa fraction as a liquid phase was suitable to understand Se sorption mechanisms on HA, since under this condition, the Freundlich isotherm independent of the solid-liquid ratio was observed. Acknowledgment: This work has been partially supported by the Agency for Natural Resources and Energy, the Ministry of Economy, Trade and Industry (METI), Japan.