The mineralization of 1.0 to 100 ng/ml of four complexing compounds--oxalate, citrate, nitrilotriacetate (NTA), and ethylene diaminetetraacetate (EDTA)--was tested in media prepared according to equilibrium calculations by a computer program so that the H, Ca, Mg, Fe, or Al complex (chemical species) was predominant. The different activities of the bacterial isolates was not a result of toxicity of the complexes or the lack of availability of a nutrient element. NTA mineralization was not enhanced by additions of Ca to Beebe Lake water, but it was enhanced when Ca and an NTA- degrading inoculum was added to water from an oligotrophic lake. The data show that chemical speciation influences the mineralization of organic compounds by naturally occurring microbial communities and by individual bacterial populations.