Several poly(alkoxyphosphazenes) and poly(fluoroalkoxyphosphazenes) were synthesized and allowed to react with various organic nucleophiles. Reactions were carried out at room temperature (25 degreesC) and at 67 degreesC in tetrahydrofuran (THF) and were monitored by P-31 NMR spectroscopy. The possible use of single-substituent polymers as hydrolytically stable macromolecular intermediates for mixed-substituent polyphosphazene synthesis is discussed, and factors that influence side group replacement in poly(organophosphazenes) are examined. In addition, evidence for a random, irreversible S(N)2-like mechanism of side group replacement is presented.