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Coupling thermal treatment with microbial reductive dechlorination is a promising remedy for tetrachloroethene (PCE) and trichloroethene (TCE) contaminated source zones. Laboratory experiments evaluated Dehalococcoides (Dhc) dechlorination performance, viability, and biomarker gene (DNA) and transcript (mRNA) abundances during exposure to elevated temperatures. The PCE-dechlorinating consortia BDI and OW produced ethene when incubated at temperatures of 30 °C, but vinyl chloride (VC) accumulated when cultures were incubated at 35 or 40 °C. Cultures incubated at 40 °C for less than 49 days resumed VC dechlorination following cooling; however, incubation at 45 °C resulted in complete loss of dechlorination activity. Dhc 16S rRNA, bvcA, and vcrA gene abundances in cultures showing complete dechlorination to ethene at 30 °C exceeded those measured in cultures incubated at higher temperatures, consistent with observed dechlorination activities. Conversely, biomarker gene transcript abundances per cell in cultures incubated at 35 and 40 °C were generally at least one order-of-magnitude greater than those measured in ethene-producing cultures incubated at 30 °C. Even in cultures accumulating VC, transcription of the vcrA gene, which is implicated in VC-to-ethene dechlorination, was up-regulated. Temperature stress caused the up-regulation of Dhc reductive dehalogenase gene expression indicating that Dhc gene expression measurements should be interpreted cautiously as Dhc biomarker gene transcript abundances may not correlate with dechlorination activity.