Abstract The structural changes of rapidly-quenched amorphous Mg–Cu–Y alloys during time-dependent room-temperature embrittlement were investigated. A continuous reduction in enthalpic content with time, corresponding to a loss in free volume, and radial atomic distances obtained from synchrotron X-ray diffraction experiments imply an overall reduction in volume. Two distinct lifetimes were found by positron annihilation lifetime spectra, with the longer-lifetime component (attributed to areas of larger free volume) vanishing after an ageing time corresponding roughly to the time of embrittlement. Coincidence Doppler broadening suggests that the immediate environment of the annihilation sites is not altered during ageing. The mechanical behaviour correlates with the structural changes in the alloy. The densification observed is assumed to influence the sharp ductile-to-brittle transition in these Mg-based glasses, and the abrupt change in bending ductility is found to result from disappearing large free volume. Keywords: metallic glassrelaxationembrittlementstructuremechanical properties Acknowledgements This work was supported by ETH Research Grant TH-21/04-2. Additional informationNotes on contributorsA. Castellero †Also at NIS Centre of Excellence, Dipartimento di Chimica IFM, Università di Torino, 10125 Torino, Italy