Aqueous two phase systems (ATPS's) composed of poly-(oxyethylene) (POE) and poly[sodium (diallylamino)methylphosphonate-alt-sulfur dioxide], a pH-responsive dianionic polyelectrolytes (DAPE) having aminomethylphosphonate motifs, have been investigated. The effects of HCl and salt (sodium chloride) on the binodal of the phase diagrams have been examined. With the increase in the concentration of sodium chloride, the binodals shifted downward, thus requiring a lower amount of polymers needed to form the ATPS's. The presence of trivalent nitrogen and negative oxygens in each repeat unit of the DAPE permitted the change of the charge types and their densities in the polymer chain, whose conformations can thus be fine-tuned for potential separation and purification of biomaterials in ATPS's. The phase separation happened at relatively low total polymer concentrations (below mass fraction of 0.10) which could be useful from the industrial point of view. The solubility behavior of the DAPE makes it a suitable component for the construction of recycling ATPS's since the polymer can be effectively removed at a lower pH by its conversion to a water-insoluble polyzwitterion (PZ) and thus recycled.