Mercury-supported hydrophilic spacers with a lipid bilayer on top are used as biomembrane models by incorporating the ion carrier valinomycin and the ion channel melittin in the bilayer. The same films supported by gold are used to incorporate valinomycin. The extrathermodynamic absolute potential difference across the mercury/water interphase is estimated from the difference in charge density between a bare mercury electrode and a mercury electrode coated with a self-assembled dioleoylphosphatidylcholine (DOPC) monolayer. This estimate is confirmed by the potential dependence of the stationary light-on current of purple membrane fragments adsorbed at the DOPC-coated mercury electrode. The surface dipole potential of a mercury-supported self-assembled monolayer of a hydrophilic spacer is estimated from diffuse-layer effects and from the previously determined value of the absolute potential difference across the mercury/water interphase. The zero potential difference across a DOPC bilayer freely suspended on top of the above hydrophilic spacer is estimated from the potential dependence of the conductance of the mercury-supported spacer/(lipid bilayer) film, upon incorporation of valinomycin and melittin. The latter measurements, besides confirming the previous estimates, are in agreement with those carried out on black lipid membranes, thus showing the validity of this mercury-supported biomembrane model. Finally, the absolute potential difference across the gold/water interphase is estimated from a comparison between the potential dependence of the conductance of mercury- and gold-supported spacer/(lipid bilayer) films incorporating valinomycin.