Marked effects of protonation (ionization) of tetradecyldimethylamine oxide on the viscoelastic properties of the micelle solutions were found. The effect strongly suggests the short-range attractive interaction between the headgroups of the nonionic (deprotonated) and the cationic (protonated) species. The zero shear viscosity reached a maximum at the half-ionized state (the degree of ionization alpha = 0.5) and the value was larger than that of the nonionic species (alpha = 0) or the cationic species (alpha = 1) by more than 2 orders of magnitude. At a surfactant concentration C of 0.1 mol/kg, approximately single Maxwell behavior was observed as a approached 0.5 from either side. For the half-ionized micelles (alpha = 0.5) in 0.1 mol/kg NaCl solutions, the steady-state compliance J(e)(0) decreased with C with an exponent of 2.1 +/-0.2, suggesting the presence of an entangled network of flexible threadlike micelles. The relaxation time, on the other hand, exhibited a nonlinear dependence on C. It was about 0.1 s and remained nearly constant in the range C > 0.1 mol/kg (regime I), whereas it increased with C in the range of C < 0.09 mol/kg (regime II) with an exponent slightly larger than 1. The single Maxwell behavior was observed in regime I. The regime shift was not controlled by the ratio C/m(s), m(s) representing the NaCl concentration. Effects of NaCl concentration and the temperature on the viscoelastic properties were also examined at alpha = 0.5. Cryo-transmission electron micrographs clearly showed a highly entangled network in the solution for alpha = 0.5, while much smaller micelles for alpha = 0. Contrary to the expectation from the theological results, a highly entangled network was also observed in the solution for alpha = 1.