In parallel with the monitoring of the distribution profiles of dissolved organic carbon (DOC) in Lake Biwa, Japan, the adsorption characteristics of DOC onto an adsorbent of hydrous iron oxide (HIO) was investigated by varying the pH of the lake water samples from 11.5 to 3.6. The first sign of the adsorption of DOC appeared around pH 8.5 ; the adsorption percentage increased sharply as the pH decreased and reached a constant value around 60% at pH 4-6, followed by a rapid decrease to 0% below pH 4.0. By utilizing the flat region (pH 4-6) of the adsorption percent, the total DOC (tot-DOC) was separated into two fractions ; the adsorption active (act-DOC) fraction and the inert (int-DOC) fraction. The latter showed a steady concentration around 0.4 mg Cl^-1 in the lake throughout a one-year period, whereas the former showed variations between 0.7 and 1.2 mg Cl^-1 depending on the season. Act-DOC was found to be reactive with permanganate ion, and the amounts of permanganate consumed for the reaction can be described as follows : the chemical oxygen demand (COD) (mg O₂l^-1)=0.97×act-DOC (mg Cl^-1)-0.50, where COD is the concentration of an equivalent amount of oxygen demanded for the oxidation. The chemical characteristics and the origin of act-DOC are discussed in light of the values of the slope and intercept of the linear relation. The int-DOC fraction had no such clear relation to COD and appeared to remain as a refractory fraction of organic carbons occurring in the lake.