This article describes a demonstration of a solvent extraction process for removal of Cs-137 from alkaline high-level radioactive waste (HLW) solutions from the Savannah River Site (SRS) tank farm. The process employed a calix[4]arene-crown-6 extractant dissolved in an inert hydrocarbon matrix, along with an alkylphenoxy alcohol modifier and an additional additive, trioctylamine, to improve stripping performance and mitigate the effects of organic anions present in the feed stream. Testing included washing and recycle of the solvent. Tests conducted with two nonradioactive simulants of SRS tank waste verified proper hydraulic operation prior to the radioactive experiments. An additional 12-hour demonstration used simulated SRS tank waste spiked with Cs-137 to confirm proper hydraulic operation, decontamination, and volume reduction factors following installation of the equipment in a shielded cell facility. Finally, a 48-hour demonstration was performed employing a composite waste from SRS tank farms. Approximately 106 L of waste (1.5E + 09 d/min/mL Cs-137 activity) was processed through the test unit. Average Cs-137 decontamination factors for the decontaminated waste streams in all tests exceeded the target value of 40,000. Concentration factors in the strip section agreed to within 5% of the target value of 15.