The role of organic matter in mechanisms of base metal transport during Mississippi Valley-type mineralization is not well understood. Evidence from field, experimental, and theoretical studies suggests that ore metals may be transported as organic complexes in Mississippi Valley-type ore fluids. However, specific ligands capable of concentrating significant quantities of lead and zinc in these ore solutions have not been identified. As a preliminary evaluation of metal transport by metal-organic complexing, activities of lead and zinc complexes involving nine carboxylate ligands (acetate, propionate, n-butyrate, phthalate, oxalate, tartronate, malate, D-tartrate, and salicylate) have been calculated for PbS- and ZnS-saturated, average ore solutions (those proposed by Giordano and Barnes, 1981) at 100 degrees and 200 degrees C. Calculated concentrations of lead and zinc in all of the complexes considered are well below the 10 ppm (10 (super -4) ) molal) minimum required to form an ore deposit.To ascertain the nature of other organic ligands which may have contributed significantly to base metal transport during Mississippi Valley-type mineralization, speciations of acetate, phthalate, and salicylate were calculated for three different average ore fluids at, or near, 100 degrees C. Results suggest that for weakly alkaline, sulfide-rich ore solutions, organic ligands should have stability constants for 1:1 and 1:2 base metal complexes of at least 10 10 and 10 20 , respectively. Such high stabilities would not be required if actual ore fluids were more acid or more oxidized, contained lower amounts of inorganic sulfide, or contained specific organic ligands at concentrations greater than 10 (super -4) molal. In addition to carboxylate, or similar complexes, metal-organic sulfide complexes may have provided a mechanism to transport significant quantities of lead, zinc, and reduced sulfur in Mississippi Valley-type ore solutions.