A convergent approach for the incorporation of arylphosphonate moieties into the cyclotriphosphazene unit is described. The synthesis is based on the reaction of hexachlorocyclotriphosphazene with diethyl 4-hydroxyphenylphosphonate in the presence of anhydrous potassium carbonate. This reaction is characterised by substitution of the chloride atoms, which thus produces the hexaarylphosphono derivative in good yield. The product of this reaction is fully characterised, including a single X-ray diffraction study. The hydrolysis of the hexaphosphonate compound to its hexaphosphonic acid analogue is reported. All products were characterised by NMR and IR spectroscopy and mass spectrometry. The thermal stability and degradation behaviour of these new compounds were investigated by thermogravimetric analysis.