The influence of electrochemically produced polyaniline colloid dispersions on the corrosion of mild steel in 0.25 M H2SO4 and 3.5% NaCl have been investigated. The colloid dispersions were stabilized and simultaneously doped by polystyrene sulfonate and co-doped with the further addition of corrosion inhibiting anions. Results indicate that mild steel can be passivated at lower potentials in the presence of polyaniline - polystyrene sulfonate colloids co-doped with phosphate, p-toluene sulfonate, nitrate and camphor sulfonate anions in the presence of H2SO4. No significant anti-corrosion activity was noted for polyaniline - polystyrene sulfonate by itself. No anti-corrosion activity was observed in 3.5% NaCl solution. Using colloid dispersions in solution, the true anti-corrosion properties of polyaniline have been probed without competing barrier properties that can be imparted by non-conductive carrier coatings commonly used in similar studies.