An in-situ gas-measuring system (1) includes an IR photon source (10) and an IR photon detector (11). The in-situ gas-measuring system (1) has an expansion chamber (12), at which an optical element (16, 16′, 16″) is arranged. A connection element (13) provides a detachable fluid-communicating connection of the expansion chamber (12) to a gas reaction chamber (2). The IR-photon source (10), the optical element (16, 16′, 16″) and the IR photon detector (11) define an optical measuring path, which extends through the expansion chamber (12). The installation and maintenance of the in-situ gas-measuring system (1) are reduced by the features of the in-situ gas-measuring system (1).