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Processing crudes with high naphthenic acid content causes corrosion problems on the crude oil distillation units. The total acid number (TAN) is commonly used to evaluate the corrosivity of crude oils; thus for decision-making related to corrosion mitigation and control in refineries. However, the TAN only represents the number of carboxylic groups present in the crude oil and does not consider the structural characteristics of the naphthenic acids, nor their reactivity, which are highly relevant to corrosion. On the other hand, the study of naphthenic acids as fractions with specific structural characteristics should enable the identification of differences in the corrosivity of crude oil with the same naphthenic acid concentration. In this research work, the fractioning of a commercial mixture of naphthenic acids was performed using the ionic strength of their respective salts. The structural characterization of the obtained fractions was conducted using Fourier-transform infrared and mass spectroscopy, gel permeation chromatography, and nuclear magnetic resonance. Furthermore, the corrosion rate of AISI SAE 1005 steel exposed to each fraction of naphthenic acids in the temperature range between 270 and 350 ºC was determined. Based on these results, a kinetic model of parallel reactions for predicting the concentration of dissolved iron in crude oil containing a mixture of naphthenic acids is proposed and validated.