The use of biocathodes in bioelectrochemical systems (BES) for the removal of nitrate in wastewater has become a vital field of research. However, the elucidation of the underlying extracellular electron transfer (EET) fundamentals of denitrifying biocathodes is still lacking, but required for a deeper BES understanding and engineering. This study reports for the first time on the thermodynamics of microbial cathodes for nitrate and nitrite reductions using microbial microcosms isolated from a running denitrifying BES. Cyclic voltammetry showed that nitrate and nitrite reduction proceed at -0.30 V, and -0.70 V vs. Ag/AgCl, respectively, by surface associated EET sites. The biocathodes were predominantly covered byThiobacillus sp. contributing with a nitrate reductase (narG) to the major function of the microscosms. In conclusion, the EET characteristics of denitrifying biocathodes are demonstrated for the first time.