Liquid-liquid equilibrium (LLE) phase compositions and tie lines were measured for the lignin-acetone-water system in the region of LLE where the aqueous lignin purification with hot agents (ALPHA) process can be practiced. The surrounding one-phase, solid-liquid (SL), SLL, and acetone-rich LL regions were also identified, mapping out the global phase behavior for this system. Solvent compositions (on a lignin-free basis) in the coexisting solvent-rich and lignin-rich liquid phases are quite similar and the one-phase region extends to similar to 50/50 wt/wt acetone/water, indicating affinity between acetone + water with respect to the lignin polymer. This renewable pseudoternary system exhibits properties useful in isolating low-molecular-weight (MW) fractions of lignin. For example, a 45/55 acetone/water mixture at a solvent-to-lignin ratio of 3:1 can be used to isolate a lignin with a number-average MW ((M) over bar (n)) of 1000 Da at 15% yield from a feed with a (M) over bar (n) of 5300.