In the scope of 11 measurement series at two combustion facilities the flue gas before and behind an electrostatic precipitator (EP) as well as the EP fly ash was sampled simultaneously and analyzed for several organic compounds. For polychlorinated dibenzofurans (PCDF) and dibenzo(p)dioxins (PCDD), polychlorinated biphenyls (PCB) and benzenes (PCBz), polychlorinated phenols (PCPh) and polycyclic aromatic hydrocarbons (PAH) formation as well as destruction was observed. The PCDF/D homologeous groups show a reproducible increase/decrease pattern.