In this study we have applied our previously developed NMR method to study the adsorption of tetramethylammonium (TMA+) and tetraethylammonium TEA+ counterions to micelles formed by perfluorooctylsulfonate (FOS-) surfactant in water at 30C. These two counterions induce formation of threadlike surfactant structures that result in well pronounced viscoelastic properties of the solution. To selectively probe the degree of counterion binding we have used 1H and 19F NMR chemical shifts and self-diffusion coefficients that are sensitive to the Stern and diffuse double layers, respectively. The competitive adsorption of TMA+ and TEA+ was examined as a function of the TMA+/TEA+ ratio at a constant FOS- concentration of 100 mM. The two counterions were found to form Stern layer around the FOS- micelles with comparable packing; about one counterion per two micellized FOS molecules. When mixed at intermediate proportions, however, the TEA+ counterion shows preferential binding; the concentration of TEA+ in the Stern layer is found to be twice higher than that of TMA+ at equal total respective concentrations in the solution. These results are discussed in terms of counterion size and hydrophobicity and presented in parallel with those that involved the smaller and more hydrophilic lithium counterion.