Approximately 125 tons per acre of dry pressurized fluidized bed combustion (PFBC) byproducts were applied during reclamation to a 7-acre abandoned coal mine in eastern Ohio, USA. The purpose of the application was to raise the pH of the soil and allow for re-establishment of vegetation. To trace leachate derived from the PFBC byproduct, sulfur-isotope ratios (delta S-34) were measured from solid-phase materials and water samples. The delta S-34 value for the PFBC byproduct ranged from +4.6 to +4.87%. Spoil and aquifer-material samples had delta S-14 values less than +3.2%. Unsaturated-zone waters within the PFBC byproduct application area had isotope signatures representative of the byproduct, whereas similar waters from outside the application area had signatures representative of the spoil. A sulfur-isotope-mixing model indicated that as much as 75% of the sulfate in the unsaturated-zone waters in the application area was derived from PFBC byproduct leachate. Sulfate concentrations in ground water increased after reclamation (from 1,110 to 2,100 mg/l in upgradient wells and from 1,770 to 1,880 mg/l in downgradient wells); however, the sulfur-isotope data indicate that sulfate in ground water was derived from oxidation of pyrite in the mine spoil, not by the leaching of PFBC byproduct. Published by Elsevier Science Ltd.