In order to apply a polymer-based aqueous biphasic system (ABS) extraction to the paper pulping process, the partitioning behavior of six model lignin species (Indulin AC, Indulin C, Reax 85 A, Reax 825 E, Polyfon T, and Alkali lignin) were studied in PEG-2000/NaOH ABS prepared from stock solutions of 40% (w/w) PEG-2000 and increasing concentrations of NaOH. In a given salt concentration, the distribution ratios increase in the order Polyfon T < Reax 825 E < Reax 85 A < Indulin C < Alkali lignin < Indulin AT due to the degree of sulfonation. In both chemical pulping and Organosolv pulping, temperatures in excess of 120 C are needed to solubilize the lignin from the cellulose, however, the availability of data on all relevant ABS system parameters, especially phase diagrams under process conditions, is limited. Phase diagrams of PEG-2000/(NH4)(2)SO4 at higher process temperatures have thus been measured. The partitioning of phthalic acid was studied in order to understand temperature effects not only on system composition, but also on phase partitioning.