Comparative studies were conducted with benzoate, propionate, oleate, tetrabutyl orthosilicate (TBOS), and biomass as substrates for dehalogenation ofcis-1,2-dichloroethene (cDCE). All five substrates supported dehalogenation. Sufficient calcium was required to precipitate oleate and thus reduce its toxicity to the dehalogenating microorganisms. More cDCE was dehalogenated with TBOS than with benzoate, although TBOS initially had an inhibitory effect. The most efficient dehalogenation was associated with biomass, 20% of which was used for dehalogenation, even higher than the 17% obtained with propionate. The advantages and disadvantages of these organic substances for introduction into an aquifer as electron donors for in situ dehalogenation were examined in terms of efficiency of electron use for reductive dehalogenation, and method and ease of introduction into the aquifer. Benzoate and propionate are useful for recirculation systems, while TBOS, oleate, and biomass are appropriate for more passive approaches.