We examine the phase transfer of polystyrene-b-poly(ethylene oxide) (PS-PEO) polymersomes from a hydrophobic ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]), into water. The dependence of the phase transfer on the molecular weight and PEO volume fraction (f(PEO)) of the PS-PEO polymersomes was systematically studied by varying the molecular weight of PS (10 000-27 000 g/mol) as well as by varying the volume fraction of PEO (f(PEO)) between 0.1 and 0.3. We demonstrate a general boundary for the phase transfer in terms of a reduced tethering density for PEO (sigma(PEO)), which is independent of the molecular weight of the hydrophobic PS. The reduced PEO tethering density was controlled by changing the polymersome size (i.e., increased polymersome sizes increase sigma(PEO)), confirming that it is the driving force in the transfer of PS-PEO polymersomes at room temperature. The phase transfer dependence on sigma(PEO) was also analyzed in terms of the free energy of polymersomes in the biphasic system. The quality of the aqueous phase, which affects the interfacial tension of the PS membrane, influenced the phase transfer. We systematically reduced the interfacial tension by adding a water-selective solvent, THF, which has a similar effect to increasing sigma(PEO). The results indicate that the interfacial tension between the membrane and water plays an important role in the phase transfer with the corona and that the phase transfer can be controlled either by the dimensions of the polymersomes or by the suitability of the solvent for the membrane. The interfacial tension-hindered phase transfer of polymersomes in the biphasic water-[EMIM][TFSI] system will inform the design of temperature-sensitive and reversible nanoreactors and the separation of polydisperse particles according to size by tuning the quality of the solvent.