The influence of different sulfur compounds on corrosion due to naphthenic acid was studied by means of the new method FeNCOR(TM). It was found that such influence occurs after the reduction of the given sulfur compound by the cathodic reaction of the overall process of naphthenic acid corrosion. When the reduction product is H2S the formation of a potentially protective layer of FeS occurs, whereas when the reduction product is H2O, coming from the reduction of sulfoxides, the naphthenic acid corrosion is enhanced. These findings help to understand crude oil corrosivity behavior and serve as a warning for blending. (C) 2004 Elsevier Ltd. All rights reserved.