Addition of methylsalicylic acid and hydroxybenzoic acid to 20 mM aqueous solutions of cetyltrimethylammonium bromide (CTAB) causes spherical micelles to undergo a transition to wormlike micelles. Shear rheometry and cryo-transmission electron microscopy (cryo-TEM) are combined to investigate the relationship between the colloid microstructures and theology as a function of acid concentration. Cryo-TEM micrographs show that the spherical-to-wormlike micelle transition is not abrupt. Both types of micelles coexist over a range of acid concentration. When the majority of the surfactant is in wormlike micelles, the solutions exhibit viscoelastic behavior as a result of micellar entanglement. The shear and extensional viscosities of these solutions undergo flow thinning, flow thickening, and then flow thinning with increasing flow rates. We interpret the micellar transition in terms of the effect the added acid has on the ratio of the effective area of the hydrophilic head group and the effective area of the hydrophobic chain of the surfactant. The effective area of the hydrophilic head group is decreased by association with the carboxylic acid group, and the effective area of the hydrocarbon chain is increased by penetration of the phenyl moiety of the acid, Both of these factors favor changing the surfactant monolayer from spherical to cylindrical curvature. From trends we observed for 3-, 4- and 5-methylsalicylic acid and 3- and 4-hydroxybenzoic acid, we conclude that increasing the effective area of the hydrocarbon chain is an important factor in the transition mechanism.