Abstract: A specimen montmorillonite (Arizona bentonite) was treated with hydroxy-aluminum solutions of various basicity. The cation exchange sites of this clay were found not to be homogeneous in their strength of retaining hydroxy-aluminum polymers; this strength varied from very weak, with which the Al polymers were only loosely held, to very strong, with which stable interlayer Al-clay complexes were formed. When a large amount of clay was treated with a small amount of solution, the polymers were held only in positions of strong affinity; the polymers were difficult to extract with neutral salt solutions and did not change to crystalline Al(OH)3 during prolonged aging. When a small amount of clay was treated with a large amount of solution, the exchange sites were completely occupied by polymers; a portion of these polymers was easily extractable with neutral salt solutions and, during aging, changed to Al(OH)3. This contrast is significant in any attempt to compare results obtained under different experimental conditions and to correlate laboratory experimentation with the occurrence of gibbsite and interlayer Al-clay complexes in nature.