Sulfite liquor, obtained during boiling with acid and containing not only a calcium, but also a mixed calcium-sodium base, after preliminary concentration by evaporation is a supersaturated solution of calcium sulfate. Independently of the degree of replacement of the calcium ion with a sodium ion in boiling acid during the two-step method used in industry involving the neutralization of the liquor by milk of lime and ammonia, the solubility curve of calcium sulfate passes through a minimum corresponding to a pH value of 3.5 for the liquor. Upon introduction of a seeding of gypsum crystal into the calcium sulfite liquor, the supersaturation of the solution may be decreased to a significant degree. The effect of the indicated parameters on the rate of crystallization of calcium sulfate was investigated for sulfite liquors of the Kotlasskiy (calcium base) and the Solikamskiy (calcium-sodium base) cellulose-paper combines.