An extreme sensitivity of pentlandite flotation towards pH regulator at pH 10 has been documented using two types of complex nickel-copper sulfide ores in Northern Ontario, Canada. The samples studied are from Sudbury and Timmins areas and differ in their mafic and ultramafic associations and degree of dissemination. It has been shown that the pentlandite in the Timmins ore is completely depressed in the lime system at a collector dosage that readily floats the pentlandite in the Sudbury ore. The general mechanism of pentlandite depression by lime is believed to be similar to that of pyrite. However, the sensitivity of pentlandite in the Timmins ore is greatly affected by its ultramafic associations and is likely related to slime coating by serpentine minerals. The tenacity of hydrophilic calcium species in the case of the Sudbury ore is believed to be counteracted by a greater activation potential (e.g., level of Cu/Ni) and collector action. When lime is replaced by soda ash, adverse effect induced by adsorbed calcium species on pentlandite is largely eliminated. Benefit resulting from better dispersion of serpentinized species in the Timmins ore is also observed using soda ash. Chalcopyrite's strong floatability overcomes difficulties experienced by pentlandite and floats well both in the lime and soda ash systems. Pyrrhotite is less floatable than chalcopyrite and pentlandite in both lime and soda ash systems. In the lime system, pyrite depression is almost the same as pentlandite depression, but unlike pentlandite it continues to be non-floatable even in the soda ash system. (C) 2014 Elsevier B.V. All rights reserved.