In situ anaerobic reductive dechlorination, using slow-release electron donors, is emerging as an effective and sustainable (low-cost and low-maintenance) technology to remediate aquifers contaminated by chloroethenes. In the present study, we investigated the use of poly-β-hydroxy-butyrate (PHB), a fully biodegradable polymer, as a slow-release source of hydrogen and acetate for the reductive dechlorination of trichloroethene (TCE). Results of this study indicated that TCE dechlorination in PHB-amended microcosms was 2.3-times higher than in non-amended controls. This higher activity was explained by a higher H2 level in PHB-amended microcosms. As usual, acetate was the major sink (∼90%) of reducing equivalents available from PHB degradation, whereas no acetotrophic dechlorination was observed.