The reduction of six chlorinated ethenes by nanoscale iron (<100 mm diameter) synthesized by borohydride reduction was studied in order to determine reaction rates and product distributions and to gain insight into reaction pathways. The transformation of chlorinated ethenes by nanoscale iron proceeded to fully dechlorinated products (ethene and ethane), with no production of chlorinated intermediates, and the reaction rate constants for disappearance of chlorinated ethenes increased with decreasing chlorination (vinyl chloride (VC) > dichloroethenes (DCEs) > tritrichloroethene (TCE) > tetrachloroethene (PCE)). This trend suggests the reduction of chlorinated ethenes by nanoscale iron did not occur under thermodynamic control (e.g. reduction potential), rather, it proceeded via a catalytic pathway involving reactive hydrogen species. The reduction of TCE under five different conditions of initial dissolved hydrogen concentrations which varied from about 0.02 to 1.2 mM confirmed the importance of hydrogen in the reaction pathway and revealed an excellent linear correlation between rate constant and hydrogen concentration. Reduction of a chlorinated ethane (1,1,1,2-tetrachloroethane) which produced 1,1-DCE as an intermediate, showed no dependence on hydrogen concentrations but the disappearance of the ethene intermediate did. TCE reduction by commercial grade micro-sized iron samples, in contrast, showed insignificant dependence on hydrogen concentration, suggesting the nanoscale iron synthesized by borohydride reduction of iron salts is particularly suited for hydrogen utilization through a catalytic reduction of chlorinated ethenes. (C) 2007 Elsevier B.V. All rights reserved.