Abstract: Ion chromatography was applied to the determination of SO42- ion in desulfurized gypsum powder, and content of CaSO4 (in w%) was calculated from the amount of SO42- found. The sample (0.050 0 g) was dissolved with 0.5 mL of HCl (1+1) solution and the solution with the insoluble residue was made up to 100 mL in a volumetric flask. An aliquot of the supernatant was used for IC analysis. The sample solution was purified by passing through Metrosep A SUPP1 column, and the eluate was separated on the Metrosep A SUPP5 anion exchange column, with a mixed solution containing 1.8 mmol·L-1 Na2CO3 and 1.7 mmol·L-1 NaHCO3 as eluant. Inhibitory conductivity detection was adopted in the analysis. Recovery was tested by addition of SO42- standard solution at 2 concentration levels to a gypsum sample as matrix, giving values of recovery greater than 95%. Value of RSD (n=8) calculated from the analytical results obtained by analyzing a gypsum sample by the present method was less than 1%.