During the leaching of several sphalerite concentrates in acidic ferric sulfate solution, the dissolution rate of zinc decreases with time and overall conversion does not exceed 30%. The growing elemental sulfur layer on the surface of each particle was found to be the cause of this passivation. To eliminate this phenomenon, a small addition of lignosulphonate (1 g/l) into the leaching solution was found to be effective. A conversion of over 60% was obtained.