<p>Herein, we studied the reaction behavior of cellulose, Japanese beech (<i>Fagus crenata</i>), and Japanese cedar (<i>Cryptomeria japonica</i>) in an ionic liquid, 1-ethylpyridinium chloride ([EtPy][Cl]), at 120&#xa0;°C. Depolymerization occurred initially in all of the samples in a manner that formed low molecular weight compounds, followed by polymerization. The obtained polymers were black in color and ultraviolet-absorbing, and morphologically as well as chemically different from the raw materials, indicating that they were humins. These humins adsorbed cations in the order Ca²⁺ &gt; Sr²⁺ &gt; Cs⁺ &gt; K⁺ &gt; Na⁺, Li⁺. There was selectivity for adsorption of cations; cellulose-derived humins adsorbed Ca²⁺ particularly easily. The adsorptivity of humins was superior in the order cellulose-derived &gt; beech-derived ≥ cedar-derived. The cation exchange capacity of cellulose-derived humins was higher than that of soil and comparable with that of natural zeolite, suggesting that humins obtained by [EtPy][Cl] treatment can be used as cation adsorbents.</p>