Resonant soft X-ray reflectivity measurements at and near the L-3 absorption edge of sulphur have been performed on mechanically polished zinc sulphide using Indus-1 synchrotron source. A sulphur rich surface (similar to 15 nm thick) consisting of two layers with gradient electron density distribution was uniquely determined. As compared to bulk ZnS, the top layer has similar to 30-50% less electron density whereas, the intermediate layer has similar to 10-18% less electron density. Conventional hard X-ray reflectivity measurement at Cu K alpha wavelength also indicates low electron density (sulphur rich) surface of ZnS but the technique was found insensitive for unique determination of electron density distribution. Optical constants of ZnS in the soft X-ray region (100-250 eV) have been reported for the first time and were in good agreement with the theoretically reported values. (C) 2010 Elsevier B. V. All rights reserved.