The inhibition effect of alizarin red (AZR) on the corrosion of carbon steel in 2.0 M H3PO4 solutions at 298K was investigated by using electrochemical impedance spectroscopy EIS, potentiodynamic polarization and weight loss measurements. A significant decrease in the corrosion rate of steel was observed in the presence of investigated alizarin red. Potentiodynamic polarization curves revealed that this inhibitor acted as mixed-type inhibitor, affecting both cathodic and anodic corrosion processes. The adsorption of the inhibitor on carbon steel surface in 2.0 M H3PO4 was found to follow Langmuir adsorption isotherm. Thermodynamic adsorption parameters (K, DGa°ds ) of investigated inhibitor were calculated from the linear form of Langmuir adsorption isotherm.