The poly-L-lysine/adenosine-5-phosphate (PLL/ATP) system is a widely used in vitro model of LLPS. Mixing PLL and ATP solutions in an adequate ratio and concentration causes phase separation, enabling the constitution of membrane-less organelles, coacervate organelles in liposomes, the design of sequential enzymatic reactions in droplets, and the investigation of droplet formation/disappearance related to enzymatic reactions and pH sensitivity of droplets in model cell membranes. Although the experimental conditions for droplet formation have been investigated, , the LLPS phase diagram for the PLL/ATP system has surprisingly not been reported to date. Especially, high-concentration conditions like in cytosol or nuclei have not been studied, despite the relevance of the LLPS behavior in high-concentration regions such as reentrant LLPS regions. Moreover, the PLL/ATP system is a good model of phase separation of proteins and nucleic acids in living cells.