AbstractThe effects on mass‐transfer and overall mixing rates of varying impeller geometry and operating speed have been studied for flat‐bladed turbines in laboratory fermentors, in aerated aqueous solutions, and in unaerated and aerated suspensions (1.6% w/v) of paper pulp. In the absence of suspended solid, oxygen absorption rates could be correlated directly with power input. In the pulp suspension, oxygen absorption at a given power input was influenced by impeller geometry and operating speed. The data for the three‐phase system can be correlated by a dimensionless equation relating oxygen‐transfer rates and mixing times to the geometrical and operating parameters of the im