The adsorption refrigerator for automobiles reduces their environmental impact of automobiles, because it uses waste heat from the engine as its energy source. However, due to high ambient temperature of automobiles, the adsorbent for the adsorption refrigerator should have special characteristics to work in such high temperature conditions. In this study, the appropriate characteristics of adsorbent for the adsorption refrigerator for automobiles was estimated and some related results were obtained as follow. In high ambient temperature conditions, the relative humidity in adsorption stage will be smaller than that in desorption stage. Therefore the adsorption isotherm should have temperature dependency. The temperature dependency of adsorbent will be estimated from the Clausius–Clapeyron’s equation, and the P/P_s region according to the value of ΔH is clarified in high temperature conditions of automobiles.