A method is described for keeping a constant salt background during protein purification in a segmented immobilized pH gradient. It is based on an external hydraulic flow replenishing the salt loss due to combined electric and diffusional mass transport (similar to the concept of Ribes' steady-state rheoelectrolysis). Such a minimum of ionic strength might be needed for proteins which tend to precipitate and aggregate at or in vicinity of the isoelectric point. However, it is found that any salt level in the sample feed (already at 1 mM concentration) deteriorates transport of non-isoelectric proteins, because of the much larger current fraction carried by the ions themselves as opposed to proteins. In addition, high salt levels in the sample reservoir might form cathodic and anodic ion boundaries, alkaline and acidic, respectively, which might hamper protein migration and even induce denaturation. Thus, when high salt backgrounds are needed in the sample feed, external pH control should be exerted, e.g. with a pH-stat. Three parameters influence protein transport in the segmented IPG chamber: (a) cross-sectional area of the Immobiline membranes; (b) delta pI between the isoelectric protein and the contaminants and (c) salt molarity in the sample reservoir. The first 2 show a positive, the last a negative correlation.