The systems NH/sub 3//SO/sub 2//P/sub 2/O/sub 5//H/sub 2/O (A) and NH/sub 3//SO/sub 2//SO/sub 3//P/sub 2/O/sub 5//H/sub 2/O (B) at 25/sup 0/C yielded solubility isotherms that showed that the buffering action of ammonium orthophosphates provides a tremendous capacity for the absorption of SO/sub 2/. In system B, the concentration of sulfate of SO/sub 3/ was significantly less than that of SO/sub 2/, except above pH 8. The concentration of SO/sub 3/ < 5% at saturation and < pH 5.5; such a low solubility could provide a means for the removal of ammonium sulfate for sale as fertilizer before regenerating the scrubber solution when scrubbing stack gases with concentrated ammonium phosphate solutions. IR spectra and x-ray powder diffraction patterns were used to characterize a new double salt sulfite, (NH/sub 4/)/sub 5/(H/sub 2/SO/sub 3/)/sub 2/PO/sub 4/. Additional data, such as the vapor pressure of sulfur dioxide and ammonia over an ammonium phosphate scrubber solution, will have to be determined before a realistic evaluation of the systems' potential can be made.