A procedure for calculating the relevant concentrations of sulfur dioxide in an MgO-SO2-H2O system and the construction of an equilibrium-operational diagram showing the relation between c(tot) (the amount of total SO2 per unit mass of solution), c(com) (the total concentration of Mg2+ in the solution expressed with SO32- and HSO3- using the corresponding stoichiometric factors), Y (the excess of SO2 over amount necessary to form Mg(HSO3)(2) only, per unit mass of solution), pH, p(SO2) (the partial pressure of SO2), and the solubility of MgSO3 at different temperatures in the range from 298 to 323 K is described. The procedure uses the Rudzinski method for determining the pH of a complex aqueous solution and Fitter’s ion interaction model to calculate the activity coefficients in the mixture at various temperatures. The calculated concentrations match closely to those obtained by experiment. The design diagram at 323 K is used for graphical representation of the operating lines for absorption of SO2 taking place within an industrial SO2 absorption unit. Stage efficiency and SO2 removal of commercial Venturi units was computed. The number of overall gas-phase mass transfer units for each stage was found to be in the range from 1 to 3.