The thionylphosphazene cation [NSO(NPCl2)(2)](+) is a proposed intermediate in the thermal ring-opening polymerization of the cyclic thionylphosphazene NSQCl(NPCl2)(2) (1) at 165 degrees C. The attempted generation of [NSO(NPCl2)(2)](+) via halide abstraction from 1 with a variety of Lewis acids has been studied and the possible intermediacy of this species in the subsequent reactions is suggested by the products isolated. The thermal reaction of 1 with 2 equivalents of AlCl3 at 80 degrees C in 1,2-dichloroethane led to the quantitative formation of NSO(CH2CHCl2)(NPCl2)(2); reaction of 1 with Ag[BF4] at 25 degrees C afforded the sulfur(VI)-fluorinated species NSOF(NPCl2)(2) and the treatment of 1 with Ag[OSO2CF3] resulted in the quantitative formation of NSO(OSO2CF3)(NPCl2)(2); the latter species was found to cleave diethyl ether and generate NSO(OCH2CH3)-(NPCl2)(2). Ambient temperature reaction of 1 with GaCl3, AlCl3 or SbCl5 (10:1), produced the high molecular weight poly(thionylphosphazene) [NSOCl(NPCl2)(2)](n) and, in addition 12-, 18-, 24- and higher-membered macrocycles. Studies showed GaCl3 as the most effective ROP initiator, and AlCl3 the least. Subsequent reaction of [NSOCl(NPCl2)(2)](n) with n-BuNH2 yielded the hydrolytically stable poly(aminothionylphosphazene) [NSO-(NHBu){NP(NHBu)(2)}(2)](n), which possessed molecular weights in the range, M-n = 25 000-60 000; PDI = 1.26-2.88. The polymerization reaction studied represents the first example of the ambient temperature ROP of any phosphazene or heterophosphazene ring induced by the deliberate addition of an initiator. Interestingly, the concentration of monomer 1 in solution was found to have a dramatic influence on the extent of reaction and the product distribution. Concentrated solutions of 1 and GaCl3 (10:1) afforded only [NSOCl(NPCl2)(2)](n) and larger macrocycles, whereas more dilute solutions afforded [NSOCl(NPCl2)(2)](n) together with smaller macrocyclic products with lower conversions. Remarkably, dilute solutions of 1 were found to be unreactive toward GaCl3. This existence of a critical equilibrium concentration (similar to 0.15 M) below which the monomer does not polymerize has only rarely been observed for inorganic systems and is indicative of a polymerization with a small Delta H value which implies that heterocycle 1 is not very strained.