The dielectric saturation at the space-charge junction of a fixed-charge bipolar membrane is studied using the theoretical approach by Booth for the water dielectric constant and the Poisson equation for the electrical double layer at the junction. The numerical solution gives the electric field and dielectric constant profiles through the junction as well as the junction thickness as a function of the voltage drop. The water dielectric constant decreases substantially for the large electric fields that may occur at the narrow bipolar junction.