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The authors review selected protocols of sequential selective extraction procedure that are used to characterize the geochemical distribution of heavy metals in soils, wastes, and sediments. They discuss the development of earlier protocols, their modifications, and the extent to which a given protocol pertains to different conditions. Emphasis is given to the considerations that led to a choice of reagents for each step and to their order in the sequence. Published studies are used as case studies to critically evaluate the implied geochemical components of operationally defined extraction steps. Also assessed are possible effects of subsequent extraction steps and conditions on the selective dissolution of the solid components and their operational definitions.