Abstract: In accordance with the principles for transcutaneous measurement of a blood parameter, such as the partial pressure of a blood gas, an electrochemical measuring electrode device (1) is applied to the skin surface (2) of a person. The electrode device has a body (10), e.g. a thick film substrate, which is thermostated to a predetermined temperature and, furthermore, includes a sensor (11) together with a membrane (14) confining an electrolyte solution within an electrolyte reservoir (15). Furthermore, the electrode device includes a jacket having an annular metallic body (16), a metallic body (4) and a thick film substrate (8), which is thermostated to the same temperature as the body (10) of the electrode device.