Sodium carbonate and sodium sulfate are the major inorganic components in black liquor produced from alkali pulping, and these two compounds have been shown to be the major contributors to the formation of encrustations on the heat-transfer surface of black-liquor evaporators. Therefore, a determination of the variables affecting the nucleation and growth of these species is an important step in elucidating mechanisms by which such encrustations are formed. The metastable zone width for crystallization in the Na2CO3-Na2SO4-H2O system was determined as a function of the total solute concentration and the ratio of sodium carbonate to sodium sulfate. The polythermal method was used in the measurements, and supersaturation was generated by heating the system contents to increase the system temperature at a constant rate. Classical nucleation theory was combined with the metastable limit to correlate the effects of temperature and solute concentration on primary nucleation kinetics.