A quantitative analysis is presented of the hydrogen pressure that is in equilibrium with adsorbed hydrogen at a Pd cathode and discussed on the basis of a mixed-controlled Volmer-Tafel mechanism of the hydrogen electrode reaction. Experimental evidence which supports this, but not the Volmer-Heyrovsky mechanism on a smooth Pd electrode, are discussed. The effective pressure can be evaluated from the slowly decaying overpotential component appearing in overpotential decay transients. Evidence is given which indicates that the overpotential component is indeed caused by adsorbed hydrogen on the electrode surface. The effects of some catalytic poisons, which augment the equivalent pressure under otherwise the same conditions, are investigated.