Oxygen transfer from dispersed air in agitated slurries represents an important unit operation in the minerals processing industry. Gold leaching, iron precipitation and especially bio-oxidation come readily to mind. However, little engineering information is available on the effect of mineral solids on such O-2 transfer. Engineering literature on the subject, i.e. information which relates slurry density and energy input to O-2 transfer, is reviewed and compared to current experimental work. The latter was carried out with minus 50 micron sand/water slurries in a 64 l laboratory tank of standard proportions, equipped with a standard Rushton turbine and variable speed drive with torque meter. Results confirm an increasing impairment of O-2 transfer with increasing slurry density. The volumetric mass transfer coefficient k(l)a, in a IO vol.% slurry shows a decrease of 30% compared To the k(l)a in water. At 40 vol.% sand this decrease is near 60%, depending somewhat on superficial gas velocities. Power needed for equivalent oxygen transfer increases even more steeply with slurry density. Experimental results are presented and their significance for slurry bio-oxidation discussed.