The present study considers the electrochemical dissolution of pyrrhotite electrodes in 1 M HCl solution. Conventional electrochemical techniques as well as electrochemical impedance spectroscopy (EIS) and in situ Mott-Schottky analysis have been applied for surface studies during the anodic dissolution of the electrode. EIS results showed the formation of two distinct surface layers on the electrode. The first layer forms during sample preparation and is stable up to around 600 mV vs saturated calomel electrode (SCE). The second layer starts to form at high anodic potentials (>670 mV vs SCE). Electrochemically active dissolution of the pyrrhotite occurs between the formation potential of the two surface layers (600-670 mV vs SCE). Mott-Schottky plots showed that both of the layers are n-type semiconductors with quite different donor densities. Moreover, the second surface layer is less conductive than the first and thus hinders the dissolution current more effectively. Three different equivalent electrochemical circuits were modeled for different dissolution potential ranges and the model regression results was compared to the experimental results of the EIS. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3421714] All rights reserved.