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The role of terrestrial humic substances as electron shuttles in bioreduction processes has gained broad acceptance as recognition has grown that the ability to transfer electrons to humic materials is distributed widely among microorganisms in nature. A fundamental property of humic substances pertinent to their mediation of reductive transformations is the maximum moles of electron charge they can transfer to an added oxidant, a parameter for which the name Reducing Capacity has been suggested. A number of different operational definitions of this important parameter have appeared in the literature recently, leading to conflicting terminology that has not heretofore been rationalized. In this paper, we present a consistent set of independent definitions of Reducing Capacity and develop laboratory methodologies for applying them, illustrating our concepts and methods with representative International Humic …