The failure strains of E-glass fibers aged in air for up to 6 months at 50 °C/80 % relative humidity were measured in ambient air and in liquid nitrogen using the two-point bending method, and were related to changes in surface morphology as characterized by optical microscopy and scanning electron microscopy. The failure strain of E-glass fibers decreased by 1.5 % per day during the first week of aging, but then more slowly (0.01% per day) over the next 6 months, with similar trends noted for fibers tested in room temperature air and in liquid nitrogen. Small droplets form on the surfaces of fibers aged within the first week and reactions between the droplets and glass create surface defects that degrade failure strains. Over longer times, calcium carbonate crystals form on the fiber surfaces.