The inhibitor effect of 3,5-diphenyl-4H-1,2,4-triazole (DHT) on the corrosion of mild steel in hydrochloric acid solution was investigated at 30 degrees C using electrochemical and weight loss measurements. Polarisation curves revealed that this organic compound act as mixed-type inhibitor and inhibition efficiencies up to 98% can be obtained. The inhibition efficiency calculated from these techniques an in reasonably good agreement. X-ray photoelectron spectroscopy (XPS) spectra of the steel surface treated with the 1 M HCl solution containing the DHT molecules revealed their adsorption on the surface. The DHT appears to function through the general adsorption mode following the Langmuir adsorption isotherm model.