Sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy was applied to determine the sulfur forms in a calcium-rich lignite coal (L coal) and a high-sulfur bituminous coal (B coal), and to investigate their transformations during the co-pyrolysis of two coals. Calcium K-edge XANES spectroscopy was used for examining the occurrence modes of calcium in two coals and their transformation during the coal pyrolysis. Sulfur was present in L coal in the form of thioether, thiophene, sulfate and sulfonate, and in B coal in the form of thiophene, pyrite, thioether and sulfate. Most calcium in L coal was organically associated. The co-pyrolysis was found to synergistically facilitate the progressive decomposition of pyrite and also result in the virtually complete elimination of thioether from the char, with the promoted retention of sulfur as CaS and CaSO4. It was evident that the calcium in L coal was an important factor underlying the synergistic effects on the sulfur transformations during the co-pyrolysis. (C) 2013 Elsevier B.V. All rights reserved.