Pentlandite can be floated selectively from pyrrhotite by setting the pulp pH to a sufficiently alkaline value and the collector addition to a moderate level. Batch flotation tests using mixtures of minerals have been used to study this separation with particular reference to the role of particle size. It has been found that the separation efficiency is limited by poor flotability of both coarse (+ 75 mum) and fine (- 10 mum) pentlandite particles and that measures that raise the flotability of these particles, such as the addition of copper sulphate, greatly increase the recovery of pyrrhotite particles of intermediate size with the result that nearly all selectivity is lost. It is concluded that careful grinding and classification of flotation feed are crucial and that separate conditioning of coarse particles with collector deserves serious consideration. Some consequences of the poor flotability of fine pentlandite for plant practice and for conventional thinking about fine particle flotation are discussed. It is also concluded that there is little prospect of separating the minerals by adjusting the pulp potential or by exploiting their self-induced flotabilities.