Preventing the release of mercury from coal-fired power plants continues to be a challenge. The design of effective and affordable control strategies depends upon the speciation of mercury from the high temperature region of the boiler to the lower temperature environment of the stack. Both homogeneous and heterogeneous oxidation pathways play a role in determining mercury's speciation over the temperature range of coal-fired flue gas. This review explores the current state of knowledge associated with the kinetically-limited homogeneous reaction pathways in addition to the complexities associated with heterogeneous oxidation processes. In particular, oxidation pathways associated with selective catalytic reduction and precious metal catalysts are considered. In addition, adsorption mechanisms on various materials are discussed, including fly ash and activated carbon for flue gas applications and precious metals for fuel gas applications. (C) 2011 Elsevier B.V. All rights reserved.