Described herein is a procedure for calculating the relevant concentrations of sulfur dioxide in a MgO-SO2-H2O system and the construction of an equilibrium diagram slowing the relationships between total SO2, combined SO2, true free SO2, and pH. The procedure used the Rudzinski method for determining the pH of complex aqueous solutions and the modified Debye-Huckel expression for calculating activity coefficients, The calculated concentrations matched closely those obtained from Hagefeldt’s diagram and measured concentrations from an actual industrial magnesium based pulping process, The equilibrium diagram was used for the graphical representation of the mass balances of the industrial absorption process.