This study used the results of breakthrough experiments to elucidate the mechanism of cesium adsorption on carbonized rice hull and beech sawdust. The shape of the breakthrough curves and the substance eluted from the carbonized rice hull and beech sawdust were investigated for various flow speeds and concentrations of the solute through the adsorbent layer. The ratio of the Cs concentration at the column outlet (C) to that at the inlet (C0), C/C0, was calculated to evaluate the mechanism. It was found that carbonized rice hull could slowly adsorb Cs as the Cs solution passed through the fixed-bed layer. On the other hand, beech sawdust could rapidly adsorb Cs upon contact with the Cs solution. It was, therefore, suggested that the two materials adsorbed Cs through an ion-exchange reaction with potassium on their respective surfaces. Moreover, the shape of the breakthrough curve of beech sawdust changed as the Cs concentration at the column inlet was varied. This phenomenon was presumably caused by a factor other than the swelling or contraction of the adsorption layer. Further analysis revealed that the breakthrough curve of beech sawdust is indicative of an overshoot phenomenon when cesium was adsorbed by beech sawdust.