Scaling by Na2CO3-Na2SO4-type solids counts among the most common problems in black liquor evaporation. Previous studies have identified two double salts precipitating from Na-CO3-SO4 solutions: burkeite, Na2CO3·2Na2SO4 and (sodium sulphate) dicarbonate, Na2SO4·2Na2CO3. The latter is believed to be more prone to cause scaling. However, it has long been suspected that other double salts, with largely unknown properties, may form in this system. This work summarizes the results of precipitation studies in the Na2CO3-Na2SO4-(additive) system targeting the transition zone between burkeite and dicarbonate formation. At least one additional double salt has been identified by X-ray diffraction in both bulk crystals and in the scale. The solubility data and the equilibrium liquor compositions have been compared with earlier models