Flame optimization in incinerators is achievable by feedback control via kinetic modelling and on-line monitoring. On-line monitoring for process control requires a fast, selective and sensitive analytical device. In this paper a REMPI (Resonance Enhanced Multi Photon Ionization) mass spectrometer is presented which combines high sensitivity with very low cross-sensitivity. Consequently, it should be a valuable tool for both kinetic research and for on-line investigation. The sensitivity of the instrument has been increased by shifting the ionization regime dose to the continuous portion (＇＇jet＇＇) of the sample beam as opposed to the more conventional ionization within the molecular beam. A further gain in sensitivity was achieved by increasing the ionization volume which was made possible by a complete re-design of the ion source. A very low base-line level is achieved by using a geometry which prevents secondary ionization by the collision of ions/electrons with surfaces. These improvements result in an excellent performance from our apparatus which is demonstrated by the dichlorotoluene and naphthalene spectra which were obtained under a variety of conditions. For naphthalene we have now a detection limit of 5 ppt (mole fraction) for a S/N = 1. Further developments of the technique are discussed.