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Molybdenite (MoS2) is predominantly recovered from the flotation of copper porphyric ores, which is carried out at a high pH and achieved with lime, which supplies Ca ions to the flotation pulp. MoS2 is selectively floated from the Cu-MoS2 bulk concentrate while depressing the Cu minerals. In the Cu ore flotation circuit, the Cu recovery is high, but that of MoS2 is low. To delineate the loss of the MoS2 natural floatability and low recovery, the characteristics of the hydrophobic face of pristine MoS2 particles have been studied through atomic force microscopy, which revealed that the face is made up of microcrystals of MoS2 having well defined nano-edges and faces. The nano-edges render hydrophilic the faces of molybdenite particles and seem to be responsible for the low natural floatability of MoS2, the adsorption of Ca ions on MoS2 faces and the decrease of this natural floatability by Ca ions at a high pH. Both hydrophobic and hydrophilic surface fractions on the face have been determined using contact angle values determined by the captive bubble and Washburn capillary penetration methods. It was found that the hydrophobic surface fraction is only about 50%. The effect of pH and Ca ions on the natural floatability of various size fractions of MoS2 particles is presented in this study., Molybdenite (MoS2) is predominantly recovered from the flotation of copper porphyric ores, which is carried out at a high pH and achieved with lime, which supplies Ca ions to the flotation pulp. MoS2 is selectively floated from the Cu-MoS2 bulk concentrate while depressing the Cu minerals. In the Cu ore flotation circuit, the Cu recovery is high, but that of MoS2 is low. To delineate the loss of the MoS2 natural floatability and low recovery, the characteristics of the hydrophobic face of pristine MoS2 particles have been studied through atomic force microscopy, which revealed that the face is made up of microcrystals of MoS2 having well defined nano-edges and faces. The nano-edges render hydrophilic the faces of molybdenite particles and seem to be responsible for the low natural floatability of MoS2, the adsorption of Ca ions on MoS2 faces and the decrease of this natural floatability by Ca ions at a high pH. Both hydrophobic and hydrophilic surface fractions on the face have been determined using contact angle values determined by the captive bubble and Washburn capillary penetration methods. It was found that the hydrophobic surface fraction is only about 50%. The effect of pH and Ca ions on the natural floatability of various size fractions of MoS2 particles is presented in this study.