Interspecies competitive effects on the degradation of two alkyl halides (1-bromo-2-chloroethane, BCA, and 1,1,2-trichloroethane, 1,1,2-TCA) and three vinyl halides (trichloroethylene, TCE; cis-dichloroethylene, cis-DCE; and trans-dichloroethylene, trans-DCE) were investigated in column experiments using Connelly iron. The presence of vinyl halides as cosolutes diminished the removal efficiencies of all compounds investigated except TCE. The presence of BCA led to a decrease in iron reactivity toward cis-DCE as a result of interspecies competition with the accumulating degradation product ethylene. Although ethylene had an inhibitory effect on some organohalides, it reacted slowly itself, consistent with the notion that its effect stems primarily from competitive sorption to reactive sites rather than from slow regeneration of reactive sites at the particle–water interface. Competitive effects observed in dual-solute experiments could be modeled via Langmuir-Hinshelwood-Hougen-Watson (LHHW) kinetics. The LHHW parameters obtained in dual-solute experiments successfully simulated interspecies competition encountered in a multiple-solute experiment in which all five organohalides were introduced simultaneously. Our results suggest that incorporating interspecies competitive effects into the design of permeable reactive barriers used in treating mixed plumes of organohalides will greatly assist in attaining performance objectives.