The kinetics of the non-oxidative dissolution of four samples of sphalerite (ZnS) of different origin were studied. It was concluded that the dissolution is independent of the stirrer speed and is first order in [H +], and that the activation energies for the removal and deposition reactions are not sensitive to the impurity content of the solid. The rate of reaction is described by an ionic charge transfer mechanism. A large addition of Zn2+ retards the initial rate because equilibrium conditions are established, whereas the addition of H2S to the reaction system lowers the final extent of reaction. The addition of Fe (III) retards the initial rate of reaction due to an anodic shift in the potential difference at the surface-solution interface, but increases the final extent of reaction as a result of the consumption of H2S by Fe (III) to form elemental sulphur and Fe (II). The observed inhibition of the initial rate for the impure samples is explained in terms of an electron-transfer theory similar to that proposed for non-stoichiometric metal sulphides.