A total of four limestone runs were made on the 10-MW spray tower system from April 10 through May 30, 1981, including two base case runs without thiosulfate (Runs SST 102 and SST 101) and two runs with thiosulfate addition (Run SST 106 and SST 112). With an inlet SO/sub 2/ concentration of 1500 to 2300 ppM, a scrubber inlet pH of 5.3, and a slurry solids concentration of about 3.5% (Runs SST 101, SST 106, and SST 112), the addition of S/sub 2/O/sub 3//sup =/ in the form of sodium thiosulfate pentahydrate (Na/sub 2/S/sub 2/O/sub 3/.5H/sub 2/O) resulted in gypsum descaling mode of operation. At 7.6 lbs/h addition rate of Na/sub 2/S/sub 2/O/sub 3/.5H/sub 2/O during Run SST 106, the S/sub 2/O/sub 3//sup =/ concentration appeared to remain constant at about 90 ppM (10 to 200 ppM range), and the spray tower operated under slightly descaling mode. The S/sub 2/O/sub 3//sup =/ concentration was questionable for this run because of analytical problems. At 15.2 lbs/h addition rate of Na/sub 2/S/sub 2/O/sub 3/.5H/sub 2/O during Run SST 112, the S/sub 2/O/sub 3//sup =/ concentration continued to increase and appeared to level off at 2200 ppM toward the end of the run after 214 operating hours. Gypsum saturation in the scrubber inlet liquor decreased to about 10% at the end of the run, and the scrubber operated under highly descaling mode.