This paper compares the results of two separate field tests using slow and fast releasing polylactate esters to remediate chlorinated aliphatic hydrocarbon (CAH) contamination in groundwater by enhancing reductive dechlorination. The two polylactate esters generate molecular hydrogen in groundwater at different rates to serve as electron donors for reduction-oxidation reactions. At both sites, both the fast and slow releasing polylactate esters have been shown to effectively change groundwater conditions to enhance reductive dechlorination of CAHs by first reducing competing electron acceptors. However, the time required to reduce the competing electron acceptor concentrations was greatly different at the two sites - approximately one month for Site 2 compared to seven months for Site 1. After one year, elevated levels of hydrogen continue to be present within the source area at Site 1. Along with quickly providing conditions favorable for reductive dechlorination at Site 2, the high levels of hydrogen produced by the fast releasing compound also induced conditions favorable for methanogenesis. A dramatic increase in methane concentration was measured at Site 2 following the injections. At Site 1, low concentrations of hydrogen produced by the slow releasing compound only slightly increased methane levels. Hydrogen is wasted in methanogenesis because it does not assist in accelerating reductive dechlorination.