A pH-stat test was used to assess extractability of inorganic sulfur compounds from fly ashes. The tests were performed in the pH range 3 - 11. Constant pH values were maintained by addition of dilute nitric acid or sodium hydroxide solutions. The only inorganic sulfur-containing species identified in the extracts was sulfate. The method showed excellent precision: relative standard deviations of repeatability and reproducibility were in the ranges 4.67 - 8.68 % and 4.67 - 10.99 %, respectively.