The properties of the chloro-protected cobalt bis(dicarbollide) anion in the acidic form (HCCD), and in the presence of polyethylene glycol (PEG-400), are well known for the recovery of Cs and Sr from acidic radioactive streams. In the early development of HCCD/PEG extraction processes, questions were raised regarding the ability to control the concentration of PEG-400 in the organic phase due to its high solubility in the aqueous process solutions relative to HCCD or the diluent. The purpose of this study was to quantify the partitioning behavior of PEG-400 under a wide variety of relevant process conditions. PEG distribution ratios (D-PEG) were measured by equilibrium batch contacts between the organic and aqueous phases over a wide range of conditions using radiometric techniques with C-14 labeled PEG-400 to monitor the behavior of the bulk material. The results vary dramatically from 0.1 < D-PEG < 50, indicate that the PEG phase transfer kinetics are rapid, and that the aqueous phase nitric acid concentration has minimal impact on PEG solubility. The molar concentration ratio of [HCCD]:[PEG] in the organic phase has the greatest impact on PEG solubility. This ratio should be maintained at [HCCD]:[PEG] greater than or equal to approximately 6 to minimize PEG losses from the organic phase.