- Abstract This article evaluates the influence of five parameters on liposome partitioning in aqueous two-phase systems (ATPSs), composed of poly(ethyleneglycol) (PEG)/dextran (Dx), using the factorial experimental design together with a multiple regression. Mathematical models to quantify the influence of these parameters, individually and/or jointly, on liposome partitioning in ATPS were developed. The models were statistically tested and verified by experimentation. This approach was then used to define the conditions for the preferential accumulation of liposomes in the top PEG-rich phase. The models predicted a significant effect of liposome surface charge, PEG molecular weight, phase-forming polymer concentration, and phosphate ion concentration on the partition behavior of liposomes. For negatively charged liposomes, it was found that the smaller the molecular weight of PEG and polymer concentration and the larger the phosphate ion concentration, the greater the partition coefficient of the liposomes. No significant effect of pH, at the range of 6–8, on liposome partitioning was noted. This approach has led to the development of an optimal two-phase system where 90% of negatively charged liposomes accumulated in the PEG phase. In addition to the general scientific value of this research, it has a technological importance as ATPSs may be useful for removing the unentrapped drug from liposomes during their preparation for pharmaceutical applications. © 1996 John Wiley & Sons, Inc.