The kinetics of the substitution reaction of hexachlorocyclotriphosphazene, (NPCL2)(3), with phenol to synthesize the partially substituted (phenoxy)chlorocyclotriphosphazene was investigated by using triphase catalysis in an organic phase/alkaline solution. The relative reaction-rate constants and the corresponding energies, enthalpies, and entropies of activation of the sequential substitution were also estimated. The diffusional limitations involve both ion diffusion in the aqueous phase and organic reactant diffusion in the organic phase within the catalyst pellet. The particle diffusion and intrinsic reactivity limit the substitution reaction in the organic phase. The diffusion of the aqueous phase in the ion-exchange step is the main rate-limiting factor.