It is demonstrated how the presence of arsenic species in the KF . 2HF melt employed as the electrolyte in fluorine production cells greatly affects the electrocatalytic behaviour of the steel electrodes that are used as cathodes. Effects of the introduction of As2O3, AsF3 and AsF5 as the sources of catalyst poison species have been studied. Such species influence the electrode kinetics of the cathodic hydrogen evolution reaction (h.e.r.) and the adsorption of the overpotential-deposited (o.p.d.) H intermediate. Using Pt, as a model electrocatalyst surface, these effects were quantitatively studied by means of potential-relaxation transients, Tafel relations and cyclic voltammetry, the latter enabling changes of underpotential-deposited C (u.p.d.) H coverage due to the presence of As species to be evaluated. By means of simulation of the potential-relaxation behaviour, information on rate constants, coverages by o.p.d. H and As species were also derived. The presence of As at Pt cathodes suppresses the u.p.d. H adsorption and modifies the o.p.d. H behaviour and the associated pseudocapacitance. The presence and generation of As(-III) species, as AsH3, in the evolved hydrogen gas was demonstrated by means of Marsh’s test. The contact-angles of hydrogen bubbles generated in the h.e.r. at mild-steel electrode surfaces were also determined comparatively in relation to observed sluggish hydrogen bubble detachment from the electrode surfaces when As species were present. Ex situ XPS analysis confirmed the presence of As species on the electrode surface.