Thales Electron Devices GmbH (TEDG) is qualifying a novel type of ion propulsion system based on High Efficiency Multistage Plasma T hrusters (HEMPT). This so-called HEMPT Assembly (HTA) will be integrated on OHB-System AG’s SmallGEO geostationary telecommunication satellite to perform attitude and orbit control manoeuvres. One of the leading requirements of the SmallGEO Ele ctric Propulsion System is the lifetime; 4800 hours of integrated operation + 50% qualificat ion margin. In order to demonstrate the lifetime capabilities of the HEMPT technology prior to the actual lifetime qualification with the QM unit, TEDG started a 4000h endurance test (c alled ET1) with a HEMPT Thruster Module (HTM) Engineering Model unit in August 2010. All HTM components were on Engineering Model level and were representative in form and function for the later QM and FM design. The endurance test was performed in the large LVTF-1 test facility at AEROSPAZIO Tecnologie s.r.l., Siena, Italy; the HTM was mainly operated in continuous mode @ SGEO working point (44 mN with a thruster power of 1380W). The test was successfully completed in March 2011, with excellen t long-term behaviour and unique reliability; no single HTM-induced firing interrupt ion has been observed. Though the HTM was mainly operated in continuous mode, 200 cycles with nominal start-up and shut-down operational sequences have been performed, with Ontime durations ranging from 40 minutes to 2-3 days. A comprehensive post-test inve stigation has shown that the thruster discharge channel was covered by a conductive carbo n layer induced by back-sputtered graphite from chamber walls; this facility-effect is an issue for stable thruster performance. Hence, ET1 has been restarted in the ULAN facility at TEDG, using a chamber wall material with sufficiently high vapour pressure (Aluminium). This allowed reaching the nominal SGEO lifetime (4800 hours) with very stable performance of all components of the HTM, which gives high confidence in the lifetime qualification still to be performed.