A device for mounting an electrochemical sensor unit to a skin surface of a human being comprises a mounting member having a circumferential wall, an adhesive tape at a lower end of the circumferential wall for adhering the mounting member to the skin surface, and a sealing member. The sealing member provides a liquid tight seal between a lumen or measuring chamber defined between the skin surface, the mounting device and the sensor unit, the measuring chamber being normally filled with a contact liquid. Alternatively, the sealing member is provided on the sensor unit. The provision of the sealing member renders the system less vulnerable to force impacts and reduces the risk of flawed measurements due to atmospheric air in the measuring chamber. The electrochemical sensor unit may e.g. be suitable for transcutaneous measurement of partial carbon dioxide pressure (pCO2) or partial oxygen pressure (pO2).