The Pitzer method for calculating activity coefficients in aqueous electrolyte solutions has been widely applied to modeling the properties of numerous industrial systems. Its use in hot, concentrated solutions, an example being black liquor from kraft pulping process, has nevertheless been limited mainly due to a lack of suitable interaction parameters. This paper summarizes our work in implementing the Pitzer method into modeling of double salt solubilities and boiling point rise in industrial black liquors. Two solubility models: for burkeite, Na 2Co 22Na 2S0 4 and for (sodium sulfate) dicarbonate, 2Na 2CO 3Na 2S0 4 were developed based on literature and experimental data, respectively. Boiling point rise was modeled in two types of black liquor: ordinary and lignin-lean after a lignin precipitation stage. The paper also presents our study on a method for estimating new Pitzer interaction parameters in hot solutions of sodium lactate using the boiling point rise data.