An adsorbent deterioration determining apparatus can accurately determine a deterioration of an adsorbent in a relatively simple configuration. The adsorbent deterioration determining apparatus determines a deterioration of an adsorbent arranged in an exhaust system of an internal combustion engine for purifying exhaust gases. The adsorbent is capable of adsorbing hydrocarbons and moisture in exhaust gases. The adsorbent deterioration determining apparatus comprises a humidity sensor arranged at a location downstream of the adsorbent in the exhaust system for detecting the humidity of exhaust gases, and an ECU for determining a deterioration of the adsorbent in accordance with a detected output of the humidity sensor at the time a predetermined time has elapsed from a start of the internal combustion engine.