Previous studies have indicated that Al is fixed by a synthetic cation exchanger as a positively-charged hydroxy-Al polymer with an OH/Al molar ratio of 2. The present study with montmorillonite showed that this mineral also can fix Al in nearly the same OH/AI ratio but at a lower pH.Graded amounts of NaOH were added to an AICl gmontmorillonite suspension and after 6 months the clay was separated. In the solution with the highest (1.35) OH/AI ratio, the apparent CEC of the clay had decreased from 98 to 20.5 me. per 100 g. and a 15.5 A. X-ray diffraction spacing was retained at 500 °C. This sample, however, expanded to 18 A. with ethylene glycol at 25°C. The increase in fixed Al with decrease in CEC was 24.6 mg. per me.Additional evidence is presented which indicates that the CEC was reduced by positively-charged hydroxy-Al groups occupying exchange sites rather than by A1(OH) 3 blocking pathways to these sites.