Abstract: Viscosity measurements of aqueous tetradecylpyridinium bromide solutions using a capillary viscometer have been made to determine the intrinsic viscosity of the micellar solutions as a function of salt concentration (0.5—2.0 mol L~(-1)NaBr), temperature (20—40 ℃), and amphiphile concentration (3—25 g/l). From the intrinsie viscosity values 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 dy Missel et al. In both 0.5 and 1.0 mol L~(-1) NaBr solutions the parameter Kealuaed at various temperatures as well as the parameter n_w calculated as a function of amphiphile concentrtion at 30 ℃ are in good agreement with those obtained in our previous studies on TPB micellar properties by static and dynamic light scattering techniques. In addition, a constancy of K deduced from viscometric data over a suitable range of amphiphile concentrations, as required by the model, was observed at various temperatures. However, the parameter K becomes remarkably concentration-dependent and an obvious discrepancy in the determined parameters K and m_w between the two methods is observed under conditions of higher NaBr concentration (2 mol L~(-1)) which demostrates the invalidity of the model. It is therefore concluded that for amphiphile systems which display the sphere-to-rod transition behavior, the viscometric technique provides a simple, reliable approach to study micelle growth, extracting extensive information on amphiphile aggregation from a quantitative analysis of concentration-dependent intrinsic viscosities.