The formation of crystals from solutions containing sodium carbonate and sodium sulfate is a complex process that often results in fouling of heat-transfer surfaces. Complexity results from a dependence of the composition of crystals formed on the composition of the solution, and the solution composition changes with crystallization. In the present work, experiments involving evaporation at 115 degreesC were used to identify these species and the specific regimes in which they are formed. Crystals produced in these experiments were sodium sulfate, burkeite, a new species referred to as dicarbonate, and either sodium carbonate monohydrate or anhydrous sodium carbonate.