Scaling caused by Na2CO3-Na2SO4 double salts constitutes a severe problem in black liquor evaporators. The ability to predict the solubility and solid-phase composition of these species would be conclusive in minimizing the adverse effects of scaling. Experimental solubility data for (sodium sulfate) dicarbonate, a carbonate-rich double salt identified in Na2CO3-Na2SO4 precipitates, were generated and used for fitting an empirical solubility model. A thermodynamically based approach for predicting the solubility and solid-phase composition, previously used for burkeite precipitates, was applied to solid solutions of dicarbonate.