Tri-n-butyl phosphate (TBP) and tri-sec butyl phosphate (TsBP) have been compared for U/Th separation by batch extraction studies and also by carrying out counter-current liquid-liquid extraction runs with a 16-stage air-pulsed ejector mixer-settler under identical conditions. The mixer-settler runs were initially carried out utilizing all stages for extraction and scrubbing (10 stages for extraction and 6 stages for scrubbing) and later integrating extraction, scrubbing, and stripping. Integral runs consisted of five stages for extraction, four stages for scrubbing, and seven stages for stripping. Data obtained from these studies are provided in this paper and indicate that TsBP is superior to TBP for U/Th separation.