Purpose of work Hydrodehalogenation of persistent pollutants, such as the groundwater contaminants trichloroethylene and diatrizoate, are catalyzed by biogenic Pd nanoparticles. As H-2 gas supply for the dehalogenation reactions is still the limiting factor, this study examines in situ H-2 production in the cathode of a microbial electrolysis cell. In a biogenic Pd nanoparticle (bio-Pd) free microbial electrolysis cell (MEC), dechlorination of trichloroethylene (TCE) with concomitant chloride and ethane formation was achieved in the cathode compartment at a removal rate of 120 g TCE m(-3) total cathode compartment (TCC) day(-1), applying -0.8 V with a power source. When the cathode granules were coated with 5 mg bio-Pd g(-1) graphite, chloride and ethane formation increased to 151 g TCE m(-3) TCC day(-1) corresponding with a specific removal rate of 48 mg TCE g(-1) Pd day(-1). In both cases, formation of unwanted byproducts, such as vinyl chloride, was not significant. When the same setup was applied for transformation of the iodinated contrast medium diatrizoate (diaI(3)), reduction in a catalyst-free cathode of a MEC resulted in a removal of 48 +/- 9% during the first h corresponding to 3 g diaI(3) m(-3) TCC day(-1). Coating the cathodic graphite granules with bio-Pd enhanced the transformation resulting in a 93 +/- 4% removal during the first h corresponding to 6 g diaI(3) m(-3) TCC day(-1). These results suggest that MECs can produce H-2 in a sustainable way to provide an economical interesting reactant for bio-Pd catalyzed dehalogenation reactions.