Recent investigations of superfluidity by a study of the mobilities of ions in liquid He ii have been extended to the liquid under pressure. At a fixed temperature the positive-ion mobility decreases appreciably as the pressure is increased, particularly at low temperatures. At a fixed pressure the mobility increases less rapidly with decreasing temperature at higher pressures. The negative-ion mobility, smaller than that of the positive ion at zero pressure, becomes equal to that of the latter above 7 atm. In high electric fields and at high pressures, the drift velocity of the negative ions approaches a limiting value roughly equal to the Landau critical velocity for a body moving through the superfluid. The theory, which discusses the mobility in terms of ion scattering by rotons and phonons, is reviewed. It is pointed out that previously neglected effects concerned with the importance of small-angle scattering of the ion ought to be taken into account; some earlier estimates of scattering cross sections are revised accordingly. It is then shown that this theory, making use only of the known change of the roton dispersion relation with pressure, can account quantitatively for the observed pressure dependence of the positive-ion mobilities.