The mobile Jet-REMPI apparatus is an optimized variant of the conventional REMPI mass spectrometry which is known for its selectivity and on-line capability. By shifting the ionization volume from the molecular regime of the sample beam as in conventional arrangements, to the vicinity of the jet, a significant sensitivity improvement was achieved without loss in the other favorable features of this technique. The laboratory set-up was converted into a mobile unit so that the technique could be field-tested. The experimental test reactor KLEAA was developed in order to simulate grate incineration under well-defined conditions. To this end selected waste samples of up to 3 kg mass are taken as fuel and are connected to the bottom end of a pre- heated cylindrical combustion chamber which is equipped with heating elements to allow for a variable temperature profile. Measurable quantities include velocities of ignition and of combustion, mass loss rate of the sample, and pollutants along with permanent gases such as CO, CO2, O2 in the exhaust gas line. The coupling of these machines served two purposes: (a) to test Jet-REMPI under realistic conditions and to study its requirements with regard to sampling techniques, (b) to better characterize and understand the non-stationary behavior of the KLEAA reactor which is operated batch-wise. For these measurements wood diluted with suitable inert material was taken as a fuel, i.e., an essentially chlorine-free situation prevailed. Samples were drawn downstream of the filter. The Jet-REMPI measurements constitute the first field measurements carried out with this method. The monitored compounds include typical products of incomplete combustion such as phenol, benzene, naphthalene, fluorene and other compounds. As a drawback it turned out that particulates cannot be tolerated in the REMPI ion source because they lead to non-resonant contributions which are recorded as noise.