A process for producing benzene-soluble substantially linear dichlorophosphazene polymer having the formula (NPCl.sub.2).sub.x, in which x is greater than 20 and may be as high as 50,000 or higher, from a cyclic oligomer having the formula (NPCl.sub.2).sub.n, in which n is from 3 to 9, particularly the trimer (NPCl.sub.2).sub.3 or tetramer (NPCl.sub.2).sub.4, in the presence of a small but significant amount of an inorganic salt, preferably a halide of a metal such as Cr, Ni, Co, Cu, Fe, Mn or Mg which affords control of the molecular weight and avoids the formation of large amounts of gel.