Bipolar membranes (BPMs) have attracted growing interest in electrochemical and photoelectrochemical systems, as they allow the unique ability to pair two different electrolytes which can be optimized for their respective oxidation and reduction reactions. Understanding the membrane voltage at a non-extreme pH gradient (ΔpH < 14) is an important step towards practical applications for electrochemical conversions, as many (photo-)electrodes and catalysts can only operate efficiently in a limited pH range. To obtain a better understanding of the individual effects that determine the BPM voltage, a complete series of experiments measuring the actual BPM voltage as a function of the pH, salt type/concentration, flow rate and current density is needed. In this paper, we present experimental results of voltage-current relations for a BPM using 16 different pH differences, 4 concentrations, 7 flow rates and permeation of 6 different ionic species. The results show that both ion cross-over and local diffusion boundary layers play important roles in the BPM voltage. We also show that the supporting electrolyte composition plays an important role, even more important than the pH itself, which is an important parameter to realize practical application of BPMs in electrochemical cells.