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