The inhibition effect of Zenthoxylum alatum plant extract on the corrosion of mild steel in 20, 50 and 88% aqueous orthophosphoric acid has been investigated by weight loss and electrochemical impedance spectroscopy (EIS). Plant extract is able to reduce the corrosion of steel more effectively in 88% phosphoric acid than in 20% phosphoric acid. The effect of temperature on the corrosion behaviour of mild steel in 20, 50 and 88% phosphoric acid with addition of plant extract was studied in the temperature range 50-80degreesC. Results on corrosion rate and inhibition efficiency have indicated that this extract is effective up to 70degreesC in 88% phosphoric acid medium. Surface analysis (XPS and FT-IR) was also carried out to establish the mechanism of corrosion inhibition of mild steel in phosphoric acid medium. (C) 2004 Elsevier Ltd. All rights reserved.