The MEGAwatt PIlot Experiment (MEGAPIE) project was started in 2000 to design, build, and operate a liquid lead-bismuth eutectic (LBE) spallation neutron target at the power level of 1 MW. The target was irradiated for 4 months in 2006 at the Paul Scherrer Institute in Switzerland. Gas samples were extracted in various phases of operation and analyzed by γ spectroscopy, leading to the determination of the main radioactive isotopes released from the LBE. Comparison with calculations performed using several validated codes (MCNPX2.5.0/CINDER'90, FLUKA/ORIHET, and SNT) yields the ratio between simulated in-target isotope production rates and experimental amounts released at any given time. This work underlines the weak points of spallation models for some released isotopes. Also, results provide relevant information for safety and radioprotection in an accelerator-driven system and more particularly for the gas management in a spallation target dedicated to neutron production facilities.