Abstract <p>Anticorrosive protection has been studied for St3 grade steel in simulated formation waters MV1 and MV2 containing H₂S and CO₂ additives with the use of an inhibitory composition containing the reaction products of polyamines with carboxylic acids in an organic solvent applying the methods of gravimetry, potentiodynamic polarization and impedance spectroscopy. The rate of steel corrosion in a less mineralized MV1 environment is generally higher than that in MV2 both under static conditions at a room temperature and at 80°C, and in the case of stirred solutions in the presence of a hydrocarbon phase. The protective efficiency of the inhibitor in MV1 and MV2 environments containing H₂S and CO₂ at a room temperature is higher and amounts to 86–87%, whereas at 80°C it is 10–15% lower as to compare to environments with H₂S. In the presence of 10% diesel fuel under the dynamic conditions, the inhibitor protective effect amounts to 96–98% at a concentration of 200 mg/L in the case of both simulated formation waters. The impedance spectroscopy and the potentiodynamic polarization methods have shown that the inhibitory composition provides a deceleration of the cathodic corrosion process. This effect is caused by adsorption on the metal. The adsorption isotherm is determined and a change in the free adsorption energy is calculated. In the presence of the inhibitory composition, hydrogen diffusion into steel in the case of MV2 is decelerated more efficiently than it is in the case of MV1.</p>