AbstractSteady‐state rheoelectrolysis of a salt of a weak base and a weak acid, with a possible excess of base or acid, is treated theoretically. For an accurate deduction of the pH course through the region where the pure salt prevails, the treatment is based directly on the mass action equations and not on Henderson's equation. However, the conductivity contributions of the water ions H+and OH−are not taken into account, a limitation that restricts the validity of the theory roughly to the pH region between 4 and 10. The pH span that can be covered by this technique is found to be about 2.5 units bigger than the pK difference between base and acid. An exact expression for the pH gradient is deduced, and it is shown that this gradient has a maximum at the location of the pure salt and minima in the neighbourhood of the two pK values. There is no theroretical obstacle to the use of this technique for isoelectric focusing without carrier ampholy