A novel monomer, 2,2-bis-(4 ' -fluorobenzoylphenoxy)-4,4,6,6-bistspiro(2 ' ,2 " -dioxy-1 ', 1 ' -biphenylyl)] cyclotriphosphazene, was synthesized and polymerized with 4,4 ' -difluorobenzophenone as a comonomer and 4,4 ' -isopropylidenediphenol or 4,4 '-(hexafluoroisopropylidene) diphenol in N,N-dimethylacetamide at 162 degreesC for 4 h to give two series of aromatic cyclolinear phosphazene polyetherketones containing bisspiro-substituted cyclotriphosphazene groups. The structure of the monomer was confirmed by H-1, C-13, and P-31 NMR. The effect of the incorporation of the bis-spirosubstituted cyclotriphosphazene group on the thermal properties of these polymers was investigated by DSC and thermogravimetric analysis.