Dilute solution properties of poly(diphenoxyphosphazene), PDPP, in THF at 25 degrees C have been studied by size exclusion chromatography, using simultaneously multiangle light scattering and differential refractive index detectors. The anomalous elution behavior and the dependence of the dimensions, i.e. radius of gyration, of the polymer on the molecular weight are discussed. Coefficients for the scaling law and unperturbed dimensions have also been calculated. Molecular Dynamics simulations of some model compounds were performed in order to analyze the conformational characteristics of the P-N-P and N-P-N pairs of skeletal bonds. The conformational model thus obtained indicates that the PDPP chain is mainly formed by sequences of about four skeletal bonds in the trans conformation separated by one bond in cis, with negligible incidence of cis-cis orientations. The chain thus generated is very rigid so that the characteristic ratio of the unperturbed dimensions C-N increases with the number of repeat units of the chain x, up to values as high as x approximate to 800, reaching an asymptotic value of ca 30 when x --> infinity, in very good agreement with the experimental result.