The operation of electrolysis cells for fluorine production is accompanied by appreciable overvoltage effects not only at the anode but also appreciably at the steel cathode. This unexpected large overvoltage is referred to here as ’hyperpolarization’ and sets in under certain conditions of current density and temperature which can lead to temporary failure of the cell operation. In a detailed study of hyperpolarization it has been found that its onset is very sensitive to the HF content in the KF.2HF melt electrolyte, as demonstrated by a series of experiments in which the HF contents of the melt were systematically varied around the formal 1:2 KF.2HF composition ratio. Experiments at a rotating cone mild-steel electrode, and one with a solidified melt, as well as studies of temperature effects, combined with the phase-diagram, provide evidence that the hyperpolarization arises on account of mass-transport limitation associated with HF consumption. The suddenness and ’irreversibility’ of the onset of the hyperpolarization effect is directly related to HF starvation in the diffusion layer at high current-densities which, in turn, leads to a more significant effect, the onset of local solidification of the electrolyte in a thin film at the electrode interface. A computer simulation of the diffusion situation provides a firm basis for the interpretation of the origin and significance of the hyperpolarization effect.