1-Phenyl-3-methylpyrazol-5-one (HPMP) was evaluated as corrosion inhibitor for mild steel in hydrochloric, nitric and perchloric acid solutions, using weight loss measurements at room temperature. HPMP was found to retard the corrosion rate of mild steel in hydrochloric and nitric acid solutions, while it aided the corrosion of same metal in perchloric acid solution. Corrosion rate decreased linearly with degree of surface coverage, and higher values of half-life were obtained for the coupons coated with HPMP inhibitor. The thicker the film of the HPMP coating on the metal, the more protection it gave to it, giving rise to the increase in inhibition efficiency.