AbstractMeasurements have been made of the rate of mass transfer of dissolved oxygen to the walls of 5 cm diameter horizontal pipes carrying aqueous slurries of sand and iron ore, in the concentration range 10 — 20 vol % at velocities up to twice the critical value for deposition. The results which show that the suspended particles greatly increase the rate of mass transfer are discussed in terms of the analogies and are compared with the results of previous workers for heat and momentum transfer. In the case of mass transfer in systems with high Schmidt numbers it appears that there is a specific effect of the solid particles relating to the disruption of the viscous sub‐la