MePS (Mono-energetic Positron System) is part of the EPOS system (ELBE Positron Source) in the HZDR (Helmholtz-Zentrum Dresden-Rossendorf). It is one of the installations at ELBE (Electron LINAC for beams with high Brilliance and low Emittance), which supplies a 40-MeV electron beam. MePS makes use of the excellent time structure of the primary electron beam of ELBE (repetition frequency up to 26 MHz; bunch length < 5ps) to produce a pulsed, intense slow positron beam to allow positron lifetime spectroscopy. In order to avoid spurious signals, which, in other systems, are often obtained by positrons reflecting from the sample surface, a bent tube (45°) was added between accelerator and sample chamber. The MePS system has been used to study the pore system of a series of low-k dielectric layers.