Experimental trials have been undertaken to determine interfacial area for mass transfer in 5cm and 25cm diameter annular centrifugal contactors. All trials used 30% TBP in Exxsol D80 diluent, with extraction of HNO3 being studied at varying flowrates and volumetric flow ratios. Droplet size was determined using a laser reflectance probe.Results from the 5cm rig showed two different regions of operation and the value of the dimensionless constant (K) in the Kolmogoroff expression was estimated to be 0.28 for O/A ratios below 2.5 and 0.30 above 2.5. From the 25cm rig trials K was estimated to be 0.26.