Herein, we report the first LLPS phase diagram of the PLL/ATP system from low to high concentrations. The previously overlooked information on the physical behavior of LLPS systems provides new insights into the design of chemical systems related to LLSP droplets and the role of LLPS droplet formation in the biophysics and chemical biology fields. We performed microscopic assays in a silicon sheet chamber on a slide glass to observe the LLPS behavior in detail. We also examined the NaCl resistance of LLPS droplets in the PLL/ATP system, where droplets formed by oppositely charged biomolecules usually dissolve under high salt concentration. By reconstructing component gradient as across the two-phase droplet region on phase diagram in a chamber, we built a model of the nonequilibrium droplet formation/disappearance process based on direct microscopy observation. The phase diagram is also important in the preparation of antibody-containing coacervates for constructing antibody drug delivery systems (DDSs). First, we constructed a phase diagram of the PLL/ATP system by changing the concentration of PLL and ATP (Figure ) from 0 to 100 mg/mL and from 0 to 1 M, respectively, as shown in Figure . The value of vertical and horizontal lines was a final concentration of a 1:1 mixture of these two solutions. The phase diagram clearly shows the boundary of the visible LLPS droplet formation area. Figure shows photographs recorded under the conditions of each point of the phase diagram. PLL/ATP droplet formation is associated with the charge correlation between cationic protonated lysine and anionic deprotonated phosphoric acid of ATP. PLL/ATP droplets dissolve by adding ions such as NaCl; hence, it is worthwhile to investigate the phase diagram of PLL/ATP system with ions. We studied the formation of droplets in the presence of NaCl using ATP solutions containing NaCl at concentrations of 31.3, 100, 187.5, 250, and 500 mM, which were mixed with PLL solution in a 1:1 ratio to construct the phase diagrams shown in Figure . The boundary region of the droplet formation phase turned to a non-droplet phase even with only 31.3 mM NaCl, and the droplet phase region decreased as the NaCl concentration increased until showing no droplet formation at a NaCl concentration of 500 mM. These results are the first evidence for the dependence of the salt resistance of droplets on the salt concentration, indicating that the boundary area and the bottom area of the original phase diagram showed the weakest and the strongest resistance, respectively.