Electrolytic etching of copper foil at the base of cavities formed by patterned photoresist was investigated in 0.5 M : sulfuric acid solutions which either contained 40 mM benzotriazole (BTA) or were free of BTA. It was found that undercutting (metal dissolution beneath the photoresist) was minimized by the action of surface films in both solutions. It was also found that the nature of the surface films and the mechanism by which they enhanced etch anisotropy differed. In additive-free solutions, anisotropic etching was observed under conditions of applied potential and flow for which mass transfer was suppressed in the interior corner regions of cavities. Such operating conditions in additive-free solutions displayed characteristic current transients. In BTA-containing solutions, the etch profiles were highly dependent on applied potential. In contrast to the additive-free solutions, the flow conditions in BTA-containing solutions had Little effect on the current transients or on the degree of undercutting within the region of applied potential in which anisotropic etching was achievable.