In this study the effect of sulfurdioxide, carbondioxide, and ammonia gases, on the cation exchange capacity of ÇANKIRI sodium bentonite used as a binder for foundry molding sand has been investigated. Cation exchange capacity of sodium bentonite samples have been determined in a tempe- rature range of 50°C-350°C being exposed for definite periods to continious gas streams, sodium acetate method has been used to determine cation exchange capacities. It was found that cation exchange capacity decreases from 3.44 % -14.4 % for carbon- dioxide, from 6.28 % - 15.6 % for sulfurdioxide and 6.27 % - 8.7 % for ammonia at the end of 1-00 minutes exposure of gas stream in the temperature range of 50°C - 350°C. It was found that with longer times of exposure and higher temperatures, the sodium hen- tonite would gradually loose its bonding ability and reactivation process would be valuable in order to restore its properties.