Researchers at the Savannah River National Laboratory successfully demonstrated the Caustic-Side Solvent Extraction (CSSX) process flow sheet using a 33-stage, 2-cm centrifugal contactor apparatus in two 24 h tests using actual high-level waste. The CSSX process for removal of cesium from alkaline solutions is the reference process for decontamination of high-level waste (HLW) at the Savannah River Site (SRS). The solvent consists of a calix[4]arene-crown-6 extractant, an alkyl-phenoxy alcohol modifier, and trioctylamine dissolved in an inert hydrocarbon matrix. Previously, we demonstrated the solvent extraction process with actual SRS HLW supernatant solution using a nonoptimized solvent formulation. Following that test, the solvent system was optimized to enhance extractant solubility in the diluent by increasing the modifier concentration. We now report results of two tests with the new and optimized solvent. The first test used a composite of supernatant solutions from two waste tanks, and the second test used a solution derived from dissolved salt cake. Test results showed that the CSSX process using the optimized solvent reduces (CS)-C-137 in HLW supernatant solutions to concentrations below the waste acceptance criterion of 45 nCi/g for disposal as low-level waste (called "saltstone"). Waste decontamination factors as high as three million were achieved during testing. Test durations exceeded 24 h of uninterrupted operation and demonstrated hydraulic stability of the contactor array while operating with the optimized solvent. Carryover of organic solvent in aqueous streams (and aqueous in organic streams) was found to be less than 1 %. The concentration factor (i.e., the ratio of the cesium concentration in the strip raffinate to the concentration in the waste) averaged approximately 13 during both tests, slightly below the process requirement of 15. Uncertainties in process flow-rate measurement and control prevented the test from achieving the target of 15.