Ab initio quantum chemical calculations on short-chain model compounds have been used to study the conformation, valence electron density, and chain flexibility of halogen-substituted poly-(thionylphosphazenes) (PTPs) [(NSOX)(NPCl&], (X = F or Cl), which are representatives of a new class of inorganic sulfur(vI)-nitrogen-phosphorus polymers. The calculations were carried out at the closedshell Hartree-Fock level of theory using the Gaussian 92 program package. The electronic wave function was described by the 6-31G* basis set. The results show that model compounds adopt a nonplanar transcis conformation in the minimum-energy state. Based on the stable geometries of the short-chain analogues, the polymer will form a 12/5 helix in its extended conformation. Rigid rotor scans and geometry optimizations of selected rotamers were used in order to investigate the torsional mobility of the main chain of the model compounds. The flexibility of the S-N-P and P-N-P bond angles contributes significantly to the chain flexibility. The torsional barriers for rotations around bonds of the PTP backbone range from 1.5 to 3.5 kcal/mol. A change from chlorine to fluorine as a substituent on sulfur leads to lower torsional barriers and wider minima of the rotational potentials and therefore to an increased torsional mobility of the main chain. The increase in chain flexibility is consistent with trends in glass transition temperatures of the corresponding polymers. The electronic structure of the model compounds, including charge density distributions, is briefly discussed. The results indicate strong charge separations along the backbone of the polymer and in the direction of the substituents bonded to the main chain which are consistent with Dewar’s island delocalization model.