The solid-state UV photocrosslinking mechanism and the properties of dense crosslinked films composed of poly[(methylphenoxy)(phenoxy)phosphazene], poly[(ethylphenoxy)(phenoxy)phosphazene], and poly[(isopropylphenoxy)(phenoxy)-phosphazene] were investigated, where the alkyl substituent was in either the meta- or para-position. Solution-cast films containing dissolved benzophenone photoinitiator (at a concentration of 1-25 mol%) were crosslinked at either 25 or 70 degrees C. The ordering of benzophenone disappearance during polymer irradiation was methylphenoxy > ethylphenoxy > isopropylphenoxy, indicating that the rate controlling step for photoinitiator disappearance was the consumption of benzophenone, either by benzopinacole formation (with the creation of a polymer crosslink) or by reaction of a benzophenone-derived ketyl radical with a polymer macro-radical. The presence of such a ketyl adduct in crosslinked ethylphenoxy/phenoxy and isopropylphenoxy/phenoxy phosphazene films was verified by solid-state NMR. The ordering of crosslinked polymer swelling (for a given initial benzophenone concentration) when films were equilibrated in dimethylacetamide( (DMAc) was isopropylphenoxy/phenoxy > ethylphenoxy/phenoxy > methylphenoxylphenoxy, indicating that steric effects of the alkyl group were playing a role during crosslink formation. The methylphenoxy/phenoxy phosphazenes were the best materials for crosslinking; the glass transition temperature increased by approximately 25 degrees C (from -15 to 10 degrees C) and the film swelling (in DMBc) decreased from infinity (complete solubilization) to 35% as the benzophenone concentration was increased from 0 to 25 mol%.