A semicontinuous slurry- microcosm method was applied to mimic trichloroethylene (TCE) cometabolic biodegradation field results at the Que-Jen in-situ pilot study. The microcosm study confirmed the process of aerobic cometabolism of TCE using toluene as the primary substrate. Based on the nucleotide sequence of 16S rRNA genes, the toluene-oxidizing bacteria in microcosms were identified, i.e. Ralstonia sp. P-10 and Pseudomonas putida. The first-order constant of TCE-degradation rate was 0.5 day(-1) for both Ralstonia sp. P-10 and P putida. The TCE cometabolic-biodegradation efficiency measured from the slurry microcosms was 46%, which appeared pessimistic compared to over 90% observed from the in-situ pilot study. The difference in the TCE cometabolic-biodegradation efficiency was likely due to the reactor configurations and the effective time duration of toluene presence in laboratory microcosms (1 days) versus in-situ pilot study (3 days). The results of microcosm experiments using different toluene-injection schedules supported the hypothesis. With a given amount of toluene injection, it is recommended to maximize the effective time duration of toluene presence in reactor design for TCE cometabolic degradation. (C) 2007 Elsevier B.V. All rights reserved.