The effects of total negative layer charge and heat treatments on Cs fixation by layer silicate minerals have been examined. High charge vermiculites produced by K-depletion of micas fixed as much as 97.8% while illites interstratified with low charge montmorillonite layers fixed only 3.9% of the total Cs sorbed from an intermediate-level simulated radioactive waste. Cs fixation increased with an increase in the total negative layer charge of the layer silicate minerals. Heat treatment to various temperatures after Cs sorption as well as exchange saturation increased the Cs fixation in all the layer silicate minerals. However, the temperature to which a layer silicate mineral needs to be heated in order to fix the Cs completely depends also on the total negative layer charge of the mineral.