Polyphosphazene derivatives have been prepared which contain monomethyl-poly(ethylene oxide) side-groups in addition to glycine ethyl esters or 2-ethyl-(O-glycyl)lactate substituents. Differential scanning calorimetry (d.s.c.) showed that polyphosphazene derivatives with poly(ethylene oxide) side-groups of sufficient length were partially crystalline. The rate of degradation of the polyphosphazene derivatives was investigated by means of h.p.l.c.-analysis. The rate of degradation could be controlled by variation of the amount of poly(ethylene oxide) substituted, or variation of the molecular weight of the poly(ethylene oxide) side-groups. Introduction of a small amount of hydrolizable groups had however a more pronounced effect on the degradation rate.