We have studied the contribution of the salt electrolyte to the electrical conductive characteristics of a bipolar membrane. We present first a critical analysis of previous theoretical approaches, and discuss the limits of validity. Experimental current-voltage curves of several commercial bipolar membranes in the low current density region are also shown. Special attention is paid to the measuring procedure and the data reproducibility. The effects of temperature, concentration of the salt electrolyte and flow rate are also considered. The comparison of theory and experiment shows the crucial role of the salt ions on the electrical conductive properties of the membrane, even when the water splitting phenomenon occurs.