The kinetics of trichloroethene (TCE) reductive dechlorination mediated by humic-Ni complexes in homogeneous aqueous solution using titanium(III) citrate as the bulk reductant was examined under various environmental conditions (e.g., pH and ionic compositions). Using Ca2+, Zn2+, and Hg2+ ions to vary Aldrich humic acid (HA)-Ni complex concentrations, pseudo-first-order rate constants for TCE reduction were observed to be proportional to HA-Ni levels (as calculated by speciation modeling), confirming HA-Ni complexes as the probable active mediator species. TCE reduction by HA-Ni was observed to be strongly pH dependent and could be due to both the variations of HA-Ni concentration and Eh with pH. Evidence is presented which suggests that quinone moieties may not be crucial for the humic-Ni mediated reduction of TCE. A variety of natural soil and aqueous humic material and Ni systems were examined, and some showed reactivity toward TCE. Humic-metal complexes may be important electron-transfer mediators in natural systems.