The possibility of precipitating high molecular weight (MW) softwood hemicelluloses has been investigated. Solids were precipitated from a sodium-based spent sulfite liquor with the anti-solvents acetone, ethanol and methanol and the effects of solute concentration, pH and temperature on the precipitation were studied. The product yield, degree of separation, MW and structure of the different fractions were determined using gravimetric analysis, high performance liquid chromatography (HPLC), ultraviolet–visible (UV-Vis) spectroscopy, size exclusion chromatography (SEC), and two-dimensional nuclear magnetic resonance (2D NMR). A direct correlation was found between the dielectric constant (also called relative permittivity) of the bulk solution and the yield of precipitants, based on which the separation of the solutes was predictable. The highest yield and degree of separation observed was an intercept around 76% with 47.5% acetone, which was the most efficient anti-solvent.