Abstract Thermogravimetric analysis (TGA) of microbial biofilms on bare and polyethyleneimine (PEI) surface modified carbon foam is described. PEI-modified carbon foam was incorporated into a microbial fuel cell (MFC) as the anode, inoculated with electrogenic bacteria, and the voltage and power outputs were monitored over five weeks. The results were compared to MFCs containing unmodified carbon foam anodes. Biofilm formation was investigated by scanning electron microscopy and TGA. TGA is presented as a new method to assess the relative amounts of biofilm on an electrode surface.