Recent laboratory and field-scale experiments demonstrated the potential for brominated industrial solid waste from biomass combustion (Br-Ash) to be an efficient, cost-effective alternative to activated carbon for capturing mercury from coal-fired power plants. To develop this attractive alternative technology to a commercially sustainable level, a better understanding of mercury capture mechanisms by Br-Ash is required. For this purpose, X-ray absorption fine-structure (XAFS) spectra of Br-Ash were collected at the Hg L(III)-edge, Br K-edge and S K-edge, and analyzed to determine the local bonding environment of mercury atoms. The coordination environment of Hg was compared with that on a commercial brominated activated carbon. Our results indicate that the mercury was captured by chemisorption on both the commercial and biomass ash sorbents; however, the mercury binding environment was different for each sorbent. Mercury was found to bind to the reduced sulfur by the commercial brominated activated carbon, in contrast to mercury binding with carbon and bromine on the brominated biomass ash. Based on the results obtained, a mechanism of Hg capture involving oxidation of elemental Hg followed by binding of the oxidized mercury on the surface of the sorbent near Br was proposed for the brominated biomass ash.