The chain flexibility of halogenated poly(thionylphosphazenes) (PTPs) [(NSOX) (NPCl2)2]n (X = F, Cl) was investigated by measuring the 31 P spin−lattice relaxation times of PTP melts. T1 times were obtained at two different magnetic fields and at various temperatures above the glass transition of the polymers. The minimal T1 time occurred at lower temperatures for the fluorinated polymer than for the chlorinated polymer. This result is in agreement with trends in glass transition temperatures of these polymers and with ab initio molecular orbital calculations on the chain flexibility of model compounds of these polymers. The Hall−Helfand model for the dynamics of polymer chains was used to fit the experimental T1(T) curves. This model describes the motions of the polymer chains that lead to the longitudinal relaxation as correlated conformational jumps. T1 measurements at two different field strengths indicated that chemical shift anisotropy contributes significantly to the spin−lattice relaxation.