Corrosion and corrosion inhibition of carbon steel in various types of crude oils and associated water (formation water) have been studied using electrochemical and weight loss measurements. The corrosion rate of carbon steel depends critically on the types of formation water The influence of three amino ethyl imidazoline derivatives, aminoethylimidazoline of linolenic acid, aminoethylimidazoline of oleic acid, and aminoethylimidazoline of propionic acid on the corrosion of carbon steel in these aggressive solutions was studied at different concentrations and temperatures. The inhibition efficiency of these inhibitors decreases in the order: aminoethylimidazoline of linolenic acid > aminoethylimidazoline of oleic acid > aminoethylimidazoline of propionic acid. The adsorption of imidazoline derivatives obeys the Langmuir adsorption isotherm. The thermodynamic parameters indicate that imidazoline derivatives inhibitors are physically adsorbed on the carbon steel surface. The influence of oleic imidazoline inhibitor was studied in fluids having different oil/water ratios. The data revealed that the corrosivity of an oil/water fluid generally increases with the increase of water cut.