A process for producing in liquid phase, concentrated sulfuric acid in the liquid phase from a source of sulfur dioxide, is described. The process comprises converting the source of sulfur dioxide to sulfuric acid. The liquid phase comprises: a) nitric acid at a sufficient concentration to commence conversion in the presence of oxygen, of sulfur dioxide into sulfuric acid in the initial absence of sulfuric acid, b) nitric acid at a sufficient concentration to maintain conversion of sulfur dioxide in the presence of sulfuric acid either generated by said conversion or present at commencement of said conversion, and c) combined concentration of nitric acid and sulfuric acid ranging from 5M to 16M where the concentration of nitric acid is at least 5M in the initial absence of sulfuric acid, and a source of oxygen is introduced to the aqueous phase at a concentration sufficient to provide for such catalytic conversion of SO2 to H2SO4.