The electrochemical and dissolution behaviour of natural pyrrhotite in 1 M HCl and 0.05 M Na₂B₄O₇ have been studied using cyclic voltammetry and potentiostatic techniques. The formation of a massive non-equilibrium metal-deficient layer (NL) has been found to occur in 1 M HCl within the potential range − 0.08 to 0 V (s. Ag/AgCl), in which the dissolution rate changes by about two orders of magnitude. The cyclic voltammograms of pyrrhotite with the NL previously formed reveal an intense cathodic peak at − 0.2 V and, when the sweep is initiated in the positive-going direction, several anodic features, with the oxidation charge passed contributing to the reduction charge on the return sweep. The cathodic peak has been argued to correspond to the reductive split of the polysulfide with the production of monosulfide S₀²⁻ centres which rapidly dissolve in acid with H₂S release. The three anodic maxima have been attributed to the association of the S₀²⁻ species with each other and with polysulfide clusters; the oxidation of the terminal S atoms (S₁⁻), with the chains uniting to form larger ones; and the oxidation of residual S₁⁻. The behaviour of the electrode with the NL created previously in hydrochloric acid and transferred into 0.05 M Na₂B₄O₇ solution agrees with these assumptions. The reactions of unetched pyrrhotite involve the same centres in lesser concentrations. The reaction mechanisms have been supposed to include the transfer of electron couples ia the lone-pair S 3p-like orbitals and fast atomic rearrangement, similar to the reactions of amorphous chalcogens and chalcogenides.