Commercial E-glass fibres drawn from melt surfaces were aged in humid air (50°C/80% RH air), dry air (32°C/ < 1%RH%), and vacuum (50°C/9 kPa) for up to 90 days and their failure characteristics were measured in air using the two-point bending (TPB) technique. Failure strains degrade more quickly when fibres are aged under humid conditions, but the dynamic fatigue parameter shows no dependence on aging conditions. The failure response is discussed in terms of surface defects characterized by scanning electron microscopy and by changes in the fibre surface energy with ageing conditions.