We report here the first examples of poly(organophosphazenes) with side groups derived from the purines guanine and adenine and the pyrimidine cytosine. Polymers with these purines or pyrimidines as the only side groups proved difficult to synthesize by macromolecular substitution techniques because of the insolubility of the intermediate products. Therefore, cosubstitution reactions of the poly(dichlorophosphazene) with glycine ethyl ester, alanine ethyl ester, or diethylene glycol methyl ether, followed by the respective purine or pyrimidine, were utilized. Each pair of side groups was incorporated into the polyphosphazene in a 1:1 ratio. P-31 NMR spectroscopy verified the replacement of all the chlorine atoms, while H-1 and C-13 NMR techniques confirmed the presence and ratio of the different side group. DRIFT spectroscopy indicated that the attachment of the purines or pyrimidines was via the primary amino functionality. Glass transition temperatures ranged from -28 to - 15 degrees C for the mixed-substituent polyphosphazenes with both purine or pyrimidine side groups together with glycine ethyl ester or alanine ethyl ester units. Other mixed-substituent polymers with purine- or pyrimidine-substituted polyphosphazenes with the cosubstituent diethylene glycol methyl ether yielded block type side group distributions, which were revealed by P-31 NMR and DSC methods. The polymers appeared to be thermally stable to at least 200 degrees C, with 10% mass loss at temperatures as high as 263 degrees C. The hydrolysis of the amino acid ester/purine or pyrimidine cosubstituted polyphosphazenes in deionized water at 37 degrees C followed a bulk hydrolysis profile with the initial reaction being a cleavage of the amino acid ester units from the skeleton, followed by subsequent detachment of the purine or pyrimidine. Significant hydrolytic weight loss and molecular weight declines were detected after 14 days for the amino acid/purine or pyrimidine cosubstituted polyphosphazenes.