This paper studies several kinetic aspects of semi-batch flotation of celestite and calcite in a mechanical or subaeration cell with sodium oleate as the collector agent, using pure minerals ranging in size between 177 and 74 mu m, respectively, with a constant pH = 9 in the flotation bath in all experiments. The results obtained indicate that both minerals float rapidly and that the flotation rate constant responds to a non-integral order equation. An increase in collector agent concentration also tends to level out flotation rates in both minerals.