In order to understand the origin of the corrosive behavior presented in the Atmospheric/Vacuum Distillation Unit in the Salina Cruz Refinery, Mexico, where a 90/10 Isthmus/Maya crude blend is processed, distributions of sulfur content and total acid number along the distillation curves of the Maya and Isthmus crude oils were performed. Naphthenic acids characterizations for both crude oils are also presented. According to our results, the Isthmus crude oil may be more corrosive than the Mayan crude oil, apparently due to the presence of low molecular weight naphthenic acids and very low sulfur contents observed in the fractions distilled at temperatures below 200 degreesC. The occurrence of this type of acids and the lack of a protective sulfur film might be the reason for this behavior. Nevertheless, other factors like salt content or equipment metallurgy should be considered before giving a final conclusion. (C) 2004 Elsevier Ltd. All rights reserved.