A physiological sensing device for the measurement of pCO2 includes a closedchamber bounded, at least partially, by a carbon dioxide permeate e membrane(12). There are two electrodes (10) within the chamber. The chamber contains asubstantially electrolyte-free liquid in contact with electrodes (10) and themembrane (12). The liquid contains a non-ionic excipient in order to preventegress of water due to an osmotic gradient across the membrane (12) in use.