A fission product solvent extraction technology for the simultaneous extraction of Cs and Sr from acidic tank waste has been developed as a collaborative effort of the Idaho National Engineering and Environmental Laboratory (INEEL) and the Khlopin Radium Institute in St. Petersburg, Russia. The process is being developed as a potential method for treating the current five million liter inventory of acidic tank waste stored at the INEEL. The fission product extraction process is based on an immiscible organic phase comprised of chlorinated cobalt dicarbollide (CCD, Cs extractant) and polyethylene glycol (PEG, Sr extractant) dissolved in a poly-fluorinated sulfone diluent. Batch contact experiments and preliminary flowsheet testing were used to define potential solvent composition and flowsheet configuration. This information was used to specify an initial flowsheet for countercurrent testing with simulated tank waste using a 3.3-cm diameter centrifugal contactor pilot plant at the INEEL. The initial extractant composition was 0.08 M CCD, 0.6 vol% PEG-400 in a phenyl trifluoromethyl sulfone (FS-13) diluent. Approximately 1.5 L of solvent was used (with continuous recycle) to treat 43 L of simulated tank waste during 75 hr of continuous operation. Greater than 99.992% of the Sr and 97.45% of the Cs were extracted from the simulated tank waste and recovered in the strip product. The matrix components Ba (>99.6%), Pb (99.8%), and Ca (10.6%) were also extracted from the simulated tank waste and recovered in the strip product. Less than 1% of the K, Na, Fe, Zr, and Mo were extracted from the tank waste simulant. Finally, none of the analytically determined waste components were observed to build up in the organic solvent.