Abstract Embrittlement of Zr 46.75Ti 8.25Cu 7.5Ni 10Be 27.5 and Zr 41.2Ti 13.8Cu 12.5Ni 10Be 22.5 bulk metallic glasses (BMGs) is studied after annealing at temperatures below and above the glass transition temperature T g for time scales comparable with structural relaxation and crystallization. The effect of annealing on the bending ductility, the isoconfigurational elastic constants, the structure and the thermal stability is examined. The embrittlement during sub- T g annealing originates from structural relaxation and can be reversed by subsequently annealing for a short duration above T g. The embrittlement kinetics correlate with the structural relaxation. However, only a fraction of relaxation time at a given temperature (< T g) is sufficient to embrittle the BMG significantly. Above T g, plasticity is retained for annealing far beyond the relaxation time but, instead, embrittlement is caused by crystallization. The magnitude of the decrease in Poisson’s ratio is insufficient to explain the severe embrittlement within the framework of a critical value as previously suggested.