Abstract: The effect of ethylene oxide-sterilization on homoionic (Na⁺, K⁺, Ca²⁺, Cu²⁺, Al³⁺, and Fe³⁺) montmorillonite samples was examined. The results indicate that ethylene oxide polymerized to polyethylene glycol in the interlayer of Cu²⁺-, Al³⁺-, and Fe³⁺-saturated clays, as substantiated by the increase in the d(001) values, as well as by the decrease in the cation-exchange capacity after reaction. On the contrary, ethylene oxide failed to react in the presence of the Na⁺-, K⁺-, and Ca²⁺-clays, likely due to the lower acidity of the exchange cations.