Regarding the difficult selective extraction of trivalent actinides over fission lanthanides from acidic media, we performed several separation tests in hollow fiber modules. Using a synergistic mixture of bis(chlorophenyl)dithiophosphinic acid and tri-n-octyl phosphine oxide in tert-butyl benzene, tests on extraction, lanthanide scrubbing, and stripping were performed. Up to 99.99% americium could be extracted from 0.5 kmol/m(3) nitric acid, with approximately one third of the lanthanides being coextracted. Mass transfer calculations using a consistent set of input data showed good agreement with the experiments. The influence of operating mode on extraction efficiency was very pronounced. This is discussed based on the calculations.