The present work revealed that the chemical polymerization of ortho-ethoxyaniline yields two types of polymers not only with different spectroscopic properties but also with different molecular weights: (1) a green form, which corresponds to the high molecular weight fraction of the polymer with a molecular weight of 800,000 g mol(-1) based on the polystyrene calibration. It is mainly composed of quinoid and benzoid structures, which is an indication of a half-oxidized polymer (emeraldine). This form of the polymer is insoluble in water-miscible solvents like ethanol and methanol and thus cannot be tested in terms of corrosion inhibition efficiency; (2) a red form, which corresponds to the low molecular weight fraction of the polymer with a molecular weight of 44,000 g mol(-1). It is composed mostly of quinold structures and exhibits an oxidation state similar to that of the completely oxidized polymer (pernigraniline). In our case, the polymer fraction, which is soluble in alcohol was first tested as a corrosion inhibitor for mild steel in acidic media, not only at conventional molecular weight (44,000 g mol(-1)) but also at different molecular weights. These different molecular weights of the polymer were obtained by adding varying amounts of neutral salt to the synthesis environment. Next, the effect of the molecular weight of the red form of the polymer on the corrosion inhibition efficiency of mild steel in hydrochloric acid solutions was investigated. The obtained results showed that the adsorption of the polymer alcoholic form obeys a Temkin adsorption isotherm with no significant change as function of inhibition efficiencies for a series of molecular weights ranging from 13,000 to 124,000 g mol(-1). The effect of temperature on the corrosion behavior of mild steel in 1M HCl with addition of 100 ppm of the alcoholic form of poly(ortho-ethoxyaniline) was studied in the temperature range 25-60degreesC. The associated activation corrosion energy was determined. (C) 2003 Wiley Periodicals, lnc.