Adsorption of Methylene Blue ( MB) on Na-, Fe- and Al-montmorillonite suspensions at 298.15 K were studied. The effect of different exchangeable cation on the adsorption amount of MB was determined. The results show that methylene blue cations( MB+) replace Na* more easily than they do Fe3+and Al3 +The adsorption isotherms on the three montraorillonites were all of Langmuir type, except that the isouHerm of MB on Na-montmorillonite presents nonmonotonic curve at low surface coverage. The specific surface areas in suspension were calculated by( MB3 +)method, and Na−is more effective to determine the specific surface area by MB method in suspension than Fe3 +and A3 +The discrepancy of the specific surface area measured by MB and BET method was discussed.