Scanning photoelectrochemical microscopy (SPECM) in ferricyanide solution was used to characterize homogeneity and electrochemical activity on nonstoichiometric iron sulfide films (FexSy) after 18 h of immersion in different media (sulphide, nitrate, sulfate, and borate). These films were prepared applying cyclic potential pulses to the carbon steel/sour medium interface [1 M (NH4)(2) S, 500 ppm CN-] for 15 min. Analysis of the electrochemical behavior and surface morphology was made using electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). In addition, SPECM was used for characterizing electronic conductivity of these films. For a FexSy film immersed in a sulfide-rich medium, neither EIS spectra nor SEM images show any important electrical and morphological modifications as compared to a film before immersion. However, SPECM images reveal significant heterogeneity of this FexSy film, suggesting surface chemical modifications. The experimental results attained by SEM regarding surface morphology after immersion in nitrate, sulfate, and borate media show good agreement with the current maps achieved by SPECM. (C) 2003 The Electrochemical Society.