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 a~kaline 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(llI)-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.INTRODUCTION THE EXPERIMENTS reported here extend those previously described on the absorption of pyrimidines, purines, and nucleosides by Li-, Na-, Mg-, and Ca-montmorillonite (Clay-Organic Studies, Part XII, Lailach et al., 1968) to montmorillonites saturated with the transition metal cations, Co, Ni, Cu, and Fe(llI). The previous work will be referred to as Part XII.Attention has been given mainly to the absorption isotherms in the pH range 2-12, and the organic compounds have been selected in part to show the influence of substituent groups on the complexing behavior of the compounds with the inorganic cations.The experimental methods for preparing the montmorillonites and measuring the organic absorptions are the same as those described in Part XII. Only for the alkaline samples of Fe(II1)-montmorillonite was a change made in the procedure for u.v.-spectroscopic measurements. Since under alkaline conditions a Fe(IlI)-hydroxide sol was formed which was not removed from solutions by centrifugation, a pH 2 buffer (HC1-KCI) was added to the supernatant and sedimentation of a gel took place within a few minutes. The clear supernatant solutions were then used for the spectroscopic measurements.