A functionalized polyphosphazene, poly[bis(carboxylatophenoxy) -phosphazene], was blended with a structural polyurethane via reactive mixing of the polymer with diisocyanate and diol prepolymers. The thermal stabilites of the resultant foams were analyzed by thermogravimetric analysis (TGA). The char yields at both 400 degrees C and 600 degrees C increased relative to the pure polyurethane upon increasing the amount of polyphosphazene from 5 wt% to 20 wt%. At higher incorporations, the char at 400 degrees C, remained the same, but the char at 600 degrees C continued to increase. The combustion behavior of these foams was analyzed both qualitatively, by a horizontal flame test, and quantitatively, by oxygen index (OI) measurements. Both of these tests indicated an increase in flame resistance at loadings of 20 wt% and above.