Protonation of oleyldimethylamine oxide (OleylDAO) decreased the average curvature and induced a change from threadlike micelles in the nonionic state to particles (vesicles) at the half-protonated state (degree of ionization alpha = 0.5), concluded from viscoelastic measurements and cryoTEM observations in water at 25 degreesC. The viscoelastic changes were confirmed to be reversible with respect to a pH change. The times required for the solutions to recover the behavior at alpha = 0.5 after the addition of HCl to the solutions of alpha = 0 were about 25 h at 0.05 mol (kg water)(-1) and 14 days at 0.15 mol (kg water)(-1). The correspondence between the viscoelastic properties and the cryoTEM observations was good. It was suggested from the viscoelastic properties that the threadlike micelles grew with alpha at alpha = 0.2, but the change from threadlike micelles to vesicles was suggested as alpha increased further. On increasing the concentration at alpha = 0.5, the vesicles became perforated, probably due to the increased counterion concentration. The present results indicate unambiguously the important and effective action of protonation in controlling the packing parameter, or the mean curvature, of amine oxide amphiphiles.