Abstract: Corrosion inhibition by alkaloids extract (AE) from Geissospermum laeve on C38 steel in 1. M HCl is investigated with electrochemical studies. Inhibition efficiency of 92% is reached with 100. mg/L of AE at 25. °C. Potentiodynamic polarization showed that the extract behaves as mixed-type inhibitors. The Nyquist plots showed that increasing AE concentration, charge-transfer resistance increased and double-layer capacitance decreased, involving increased inhibition efficiency. Adsorption of the inhibitor molecules corresponds to Langmuir adsorption isotherm. Immersion time and temperature effects were investigated using EIS and potentiodynamic polarization. SEM and EDX supported the adsorption conclusions. The active compound responsible for the corrosion inhibition is geissospermine.