Novel biodegradable polyphosphazenes containing glycine ethyl ester and benzyl ester of amino acethydroxamic acid as cosubstituents (PGBP) were synthesized by further modifying poly[bis(glycine ethyl ester)phosphazene] (PGP). The polymers were characterized by IR, H-1-NMR, DSC, and elemental analysis. Degradation experiments were conducted in vitro at varied pH conditions. The results indicated that the degradation of PGBP was pH-sensitive. The sample dissoluted after 1.5 days under a physiological condition (pH 7.4) but took more than 20 days under an acidic condition (pH 5-6), which was related to the content of the benzyl eater of amino acethydroxamic acid in the polymer. The "two-stage" degradation mechanism of PGBP was proposed: that the polymer first degraded to a water-soluble polymeric product with the fast break of side groups, followed by the relatively slow scission of the backbone. This property of PGBP may be useful in controlled drug-delivery systems.