Abstract: Polymeric aqueous two-phase systems (ATPS) are widely used in bioseparation processes. Characterizing the phase diagram of an ATPS is critical to identify working concentrations of polymers that result in two immiscible aqueous phases for such applications. We describe a new approach to computationally construct the binodal curve of an ATPS based on the mass balance of phase-forming polymers and only using empirical volume and density of equilibrated phases of several two-phase solutions from the ATPS. A major benefit of this method is that it simultaneously returns compositions of top and bottom phases of all two-phase solutions used to generate the binodal curve and, as a result, resolves tie-lines and the critical point of the phase diagram. We validate this approach by comparing it with a widely used turbidometric titration technique for constructing binodals of polymer–polymer and polymer–salt systems, the accuracy of locating the critical point on a phase diagram, and by comparing compositions of equilibrated phases with those estimated from independent fluorescent imaging of equilibrated phases. This simple method will greatly benefit fundamental and applied studies involving ATPS.