Minerals are classified into three groups according to their critical surface tension of wetting (gamma-c) values measured in methanol/water system: 1. Naturally hydrophobic minerals with gamma-c values between 26 and 35 dyn/cm (talc, sulfur, graphite, molybdenite, orpiment, realgar) which can be easily floated without using any collector 2. Partially hydrophobic minerals with gamma-c values between 35 and 49 dyn/cm (galena, sphalerite, chalcocite, chalcopyrite, stibnite). 3. Hydrophilic minerals with gamma-c values about 72 dyn/cm, these cannot be floated without first decreasing their gamma-c values to 26-35 dyn/cm by surface treatment.