The counteranion associated with the cationic initiator [Cl3P=N=PCl3](+) ([4](+)) generated during the PCl5-initiated living, cationic chain growth polycondensation of the N-silylphosphoranimine Cl3P=NSiMe3 (3) to give poly(dichlorophosphazene), [N=PCl2](n) (2), has been found to have a dramatic effect on the polymerization. When the counteranion of [4](+) was changed from PCl6- or Cl- to the weakly coordinating anions [BAr4*(F)](-) and [BAr4F](-) (Ar*(F) = 3,5-{CF3}(2)C6H3, Ar-F = C6F5) instead of the polymerization of 3 being complete in 4-6 h, no reaction was observed after 24 h. Remarkably, the polymerization of 3 may be initiated by Cl- anions even in the absence of an active cation such as [4](+). However, in the presence of [4](+), the reaction proceeded significantly faster and allowed for molecular weight control. These results reveal that the currently accepted mechanism for the PCl5-initiated living polymerization of 3 needs to be revised to reflect the key role of the counteranion present.