Structural relaxation during isothermal annealing, quantified by enthalpy recovery of Zr44Ti11Cu10Ni10Be25 towards its metastable equilibrium and correlation to embrittlement, quantified through fracture toughness, K-Q, is studied. Enthalpy relaxation over time obeys the Kohlrausch-William-Watts (KWW) stretch exponent with beta = 0.74 and tau = 11,000 s. Such beta and tau are used to fit experimental K-Q(t) with KWW, resulting in R-2 = 0.79. This finding combined with a controlled characterization of the glasses' K-Q versus temperature, fictive temperature, and their combination, revealed that embrittlement in metallic glasses is predominantly controlled by structural rearrangements, whereas volume changes from thermal expansion have negligible influence. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.