The effects of nine different anionic and cationic micelle-forming surfactants on the viscosity and viscoelasticity of semidilute solutions of hydrophobically modified hydroxyethyl cellulose (HMHEC) were studied. All added surfactants gave rise to the well-established viscosity maximum with increasing surfactant concentration, but the total surfactant concentration at the maximum, as well as the magnitude of the viscosity enhancement, varied widely between the different surfactants. A detailed analysis of the oscillatory shear data for the most viscous samples showed that added surfactant affects both the lifetime and the structure of the transient HMHEC-surfactant network, due to mixed micellization between surfactant molecules and the hydrophobic side chains of HMHEC. The viscosity maximum is the result of two opposing effects of increasing surfactant concentration: an increase in the lifetime of mixed micellar cross-links and a decrease in the number of cross-links. All data indicate that the structure of the HMHEC-surfactant network and the compositions of the mixed micellar cross-links are quite similar for the various surfactants, when the mixtures are compared at their respective viscosity maxima. The shifts of the viscosity versus surfactant concentration curves between the various surfactants may thus be rationalized in terms of differences in lifetimes of the mixed micellar junctions and in the proportions of free (monomeric) surfactant. The free surfactant concentration at a given degree of surfactant binding is proportional to the critical micelle concentration of the surfactant. The lifetime of a mixed micellar cross-link increases with surfactant chain length and with the nature of the hydrophilic headgroup according to the series trimethylammonium < ammonium approximate to diethoxysulfate < sulfate.