A sensor element for determining concentration of at least one constituent of a gas mixture operates on an electro-chemical measuring process and includes a gas measuring chamber; a pair of electrodes and an electrode chamber located between the pair of electrodes; and a diffusion conduit adjoining the electrode chamber and having a gas input which connects with the gas measuring chamber, having a gas output which connects with the electrode chamber, and having at least one cross-sectional area including a height, w, and a length, L, which height, w, has a preselected minimum value, and which length, L, has a value which is equal to or greater than the preselected minimum value of the height, w. The constituent of the gas mixture has a mean free path length through the sensor element, the gas measuring chamber has a cross-sectional area including a height and a length each of which are at least as large as the minimum height, w, of the diffusion conduit, and the height, w, of respective ones of the at least one cross-sectional area of the diffusion conduit is at least a multiple of the mean free path length of the constituent of the gas mixture to be measured. Advantageously, substantially only gaseous phase diffusion occurs when the predetermined minimum value of the height, w, is at least a multiple of a minimum value of the mean free path length for the gas constituent measured.