The exchange current density (ECD) for the reaction: Fe_{1+ y}S₂ = Fe_{1+ y−x}S₂ + xFe²⁺ + 2xe⁻ was estimated measuring the impedance of pyrite electrodes in the solutions of iron (II) sulfate. For five different pyrite electrodes, 0.5 mol dm^-3 FeSO₄ solution and potentials close to the potential of the reaction FeS₂ = Fe²⁺ + 2S⁰ + 2e⁻ the ECD was found to be between 8 and 15 μA cm². Low value of the ECD causes that the potential of pyrite electrode in solutions of iron (II) sulfate does not attain the equilibrium value (i.e. the value within the range of pyrite thermodynamic stability) except the most concentrated and very well deoxidized solutions. For the concentrations of FeSO₄ solution lower than 0.5 mol dm^-3 pyrite electrode shows the rest potential outside the limits of thermodynamic stability, which means that the measured potential is a mixed (corrosion), not equilibrium potential.