Abstract: Viscosity measurements havebeen made to determine the specificity of some counterions to induce the growth of micelles. From the intrinsic viscosity values abtained several basic micellar parameters including the thermodynamic parameter K, governing the sphere-to-rod transition in the micelle growth process, and the weight average aggregation number of polydisperse micelles, n_w, were calculated based on a thermodynamic model for micellization developed by Missel et al. The elongation of tetradecylpyridinium bromide micelles in aqueous solutions are induced by the monovalent anions Br~-, NO_3~-, ClO_3~-, the efficiency is ClO_3~->NO_3~->Br~-. In both 0.5 mol L~(-1) NaBr and KNO_3 Solutions a constancy of K was observed over a suitable range of ampnipnile concentrations. The parameter K as well as the parameter n_w calculated as a function of amphiphile concentrations are in good agreement with those obtained by static and dynamic light scattering techniques. In 0.5 mol L~(-1) KClO_3 solution, the parameter K becomes remarkably concentration-dependent and an obvious discrepancy in the determined parameter n_w between the two methods is observed, which demonstrates the invalidity of the model.