The methyl groups in both cyclic and polymeric methylphenylphosphazenes, [Me(Ph)PN]3 and [Me(Ph)PN]n, were modified using deprotonation-substitution and subsequent oxidation reactions to incorporate both thioether and sulfone functional groups. The new cyclic phosphazenes, cis-[Ph(RSCH2)-PN]3, 3 and 5, and cis-{Ph[RS(O)2CH2]PN}3, 6 and 7, where R ) Me or Ph, and the corresponding copolymers [Me(Ph)PN]x[Ph(RSCH2)PN]y, 8 and 9, and [Me(Ph)PN]x[{Ph[RS(O)2CH2]PN}y, 10 and 11, with varying ratios of x:y, were characterized by NMR and IR spectroscopy and thermal analysis.Crystallographic analysis of the cyclics 3 and 6 show that these molecules retain the basket shape of the nongeminal cis isomers with all the new functional groups opposite from the phenyl groups about the planar P3N3 ring. A cyclic trimer, cis-[Ph3(MeSCH2)2MeP3N3], 4, in which only two thioether groups had been incorporated was also isolated. The thioether-substituted polyphosphazenes, 8 and 9, were used as phase-transfer reagents and stabilizing hosts for the synthesis of gold nanoparticles. Analysis by UV spectroscopy and transmission electron microscopy indicated that the average gold particle size in these polymer nanocomposites was 3.2(0.9) nm. In solution, some aggregation of the nanoparticles was observed after one week, but the nanocomposites remain unchanged in the absence of solvent.