The formation mechanism of 1,4-difluorobenzene (5) has been studied during the electrolyses of chlorobenzene (1), 1-chloro-4-fluorobenzene (2), bromobenzene (3) and 1-bromo-4-fluorobenzene (4) in Et(4)NF . mHF. The mechanism consists of a cathodic dehalogeno-deffuorination of 3-chloro-3,6,6-trifluoro-1,4- cyclohexadiene (2a) (or 3-bromo-3,6,6- trifluoro-1,4- cyclohexadiene (4a)) which was produced by anodic fluorination of 1 and 2 (or 3 and 4). The reaction should compete with the cathodic evolution of hydrogen. The ratio of the dehalogeno-defluorination and the hydrogen evolution varied with the cathode potential, the content of HF (m) in the electrolyte Et(4)NF . mHF and the concentration of 2a or 4a in the electrolyte solution. The chloride and bromide anions produced through the cathodic dehalogeno-defluorination are anodically oxidized to chlorine and bromine radicals, respectively. The radicals would further react with the substrate compounds or the fluorinated products, so that many kinds of halogenated compounds were also produced as well as the fluorinated compounds.