Oxygen transfer measurements in agitated mineral slurries were carried out in a 67 l laboratory tank. Two impellers (a Rushton turbine and a Lightnin A315) and two types of solid particles (silica, d(50)=13 mu m, and pyrite, d(50)=14 mu m) were investigated. Next to the volumetric oxygen transfer rate k(L)a, local bubble sizes, the overall gas hem-zip in the tank, and the power input were measured The silica and pyrite slurries behaved remarkably different with respect to the oxygen transfer as a function of the solids fr action. The pyrite particles increase k(L)a, whereas silica impairs oxygen transfer. The bubble size measurements indicate that the increased k(L)a in pyrite slurries is related to an increased interfacial area in the tank.