Capillary penetration techniques were used to determine the wettability of sphalerite particles under flotation related conditions. Particle contact angles were determined separately from measurements of wetting rates (Washburn technique) and equilibrium capillary pressures across a sphalerite particle bed. For sphalerite of different size fractions, particle contact angles ranged from 75 to similar to 90 degrees, whereas synthetic ZnS (99.9%) gave values of 46 degrees and 53 degrees. The strong hydrophobic nature of the natural sphalerite samples was due to inadvertent copper activation and the ensuing formation of sulfur-rich surfaces, determined by bulk chemical and surface analysis (X-ray photoelectron spectroscopy and scanning Auger microscopy) studies. Scanning Auger microscopy also revealed variations in the levels of surface copper from mineral grain to grain and confirmed the heterogeneous chemical nature of sphalerite surfaces. The latter enables the contact angle variability to be discussed.