We have investigated the viscosity boosting effect of ionic surfactant on nonionic wormlike micellar solutions formed in the water/sucrose oleate (C18:1SE) system. The zero-shear viscosity increases one order of magnitude from that of a binary water/C18:1SE system by substituting the nonionic surfactant with ionic surfactant. The viscosity boosting effect depends on hydrophobic chain length and the ionic species of ionic surfactant. On increasing ionic surfactant composition, relaxation time increases; however, the plateau modulus is approximately constant. Zero-shear viscosity is a product of the relaxation time and the plateau modulus; hence we consider that the increase in the relaxation time is responsible for the viscosity boosting. Ionic surfactant added to nonionic wormlike micelles gives an electric double layer and it causes an electrostatic excluded Volume effect. Therefore the diffusion constant of wormlike micelles, which can relax by reptation, may decrease due to the growth of the electric double layer and thus the relaxation time increases. However, excess addition of ionic surfactant causes reduction of viscosity, This may be attributed to the geometry change of wormlike micelles to spherical micelles due to the large repulsion between the hydrophilic groups of ionic surfactant. (C) 2009 Elsevier Inc. All rights reserved.