Naphthenic acid, a significant cause of corrosion of carbon-steel in crude oil, has been investigated at elevated temperatures using vibrational spectroscopic methods (Raman and Fourier transform infrared (FT-IR)). Unlike earlier reports of studies at ambient temperatures, these elevated temperature experiments performed on a series of carboxylic acids having structures similar to naphthenic acid components in crude oil and on a commercial naphthenic acid mixture show a progressive increase with increasing temperature in the concentration of monomer over the multimers, which drives the formation of iron naphthenate. This observation forms a reasonable basis for proposing a mechanism of corrosion in crude oil at temperatures closer to the boiling point of naphthenic acids, which proceeds through the acid monomer.