Abstract: Absorptions of purine and pyrimidine derivatives by Co- and Ni-montmorillonite at pH < 6 and by Cu-montmorillonite at pH < 3 are similar to their absorption by Ca-montmorillonite and take place primarily by a cation exchange process. In the weakly acidic to weakly alkaline range, absorption is due to complex formation with the inorganic cations, and decrease in the order Cu ≫ Ni > Co ≫ Ca. Adenine, 7-methyladenine, hypoxanthine, and purine are strongly absorbed, 9-methyladenine, 6-chloropurine, and cytosine are weakly absorbed, and thymine and uracil are not absorbed. At pH < 5, the nucleosides are absorbed by Co-, Ni-, and Cu-montmorillonite in approximately the same manner as by Ca-montmorillonite, but at pH > 6 their absorptions decrease in the order Cu ≫ Ni > Co > Ca. Fe(III)-montmorillonite behaves quite differently from the other montmorillonites studied. With purines and pyrimidines, there is strong absorption from pH 3 to pH 7–8; with the nucleosides, the absorption varies considerably with the compounds considered decreasing in the order adenosine > cytidine ≫ guanosine ≫ inosine.