Abstract: Proposed is an electrochemical measuring instrument (10) for determining the oxygen content in gas mixtures, in particular exhaust gases from combustion engines, with a sensor element (14) that is installed potential-free in a metal casing (11). The sensor element (14) has an oxygen ion conducting solid electrolyte body (23) in the shape of a tube closed on one side, with electrodes (25, 26) and connections (27, 28), and is fitted inside the casing (11) with a metal sealing ring (20). The connection (27) which runs along the outer surface of solid electrolyte body (23) is covered with an electrically insulating layer (21), at least in the area surrounding sealing ring (20). In addition, a ductile cover layer (31) that spans solid electrolyte body (23) is applied over insulating layer (21) in the area surrounding sealing ring (20), with which the sensor element (14) rests on sealing ring (20).