In this work partitioning of TAMRA (Tetramethyl-6-Carboxyrhodamine) - oligonucleotide in systems containing 18% (w/w) polyethylene glycol (PEG 600, PEG 1000, PEG 1500 and PEG 4000) and 15% (w/w) sodium sulfate (designated as 18/15) was investigated. To find the optimum condition, effective parameters on biomolecule partitioning including polymer molecular weight, pH, temperature and addition of a second salt were studied. The results showed that as pH increases and molecular weight of polymer decreases, the affinity of TAMRA oligonucleotide to accumulate in the top PEG-rich phase increases. In order to find the temperature effects, partitioning of TAMRA oligonucleotide was studied at two different temperatures 20 and 30 degrees C). At this range, no significant change was observed. Meanwhile, it is found that addition of 0.5% (w/w) of KH2PO4 as the second salt reversed the partitioning behavior of the oligonucleotide. A similar trend with smaller gradients was observed when KH2PO4 was substituted by KCI; whereas, substitution by MgSO4 increased the affinity of the oligonucleotide to partition to top phase. The best results achieved using PEG 600 at pH = 6. In this case, almost 100% of TAMRA oligonucleotide was recovered in the upper phase. Finally, the partitioning of the biomolecule was correlated using a model based on UNIFAC-FV group contribution. The results showed that the model can correlate the partitioning of the oligonucleotide with an acceptable accuracy. (C) 2016 Elsevier B.V. All rights reserved.