Mechanical properties of an S2-glass/epoxy composite have been characterized at room temperature and at 100°C in vacuum. The latter was to simulate the flywheel service environment. The epoxy was DER 332/Menthane Diamine (100/24.5), and the composite was filament-wound and cured 2 h at 150°C. It has been found that the composite continues to lose weight even after ∼150 days of conditioning at 100°C in vacuum. The weight loss is accompanied by a change of color from translucent white to brown. The present composite shows less moisture absorption than most graphite/epoxy composites for aerospace applications. The 100°C/vacuum environment has only a minor effect on the mechanical properties, both static and fatigue. However, the moisture desorption in vacuum increases residual stresses and leads to ply cracking in laminates. The logarithmic fatigue life is linearly related to the fatigue stress, and the fatigue strength at 106 cycles is ∼20% of the average static strength at room temperature and also at 100°C in vacuum. However, the corresponding fatigue strength at 100°C in air is below 20% of the average static strength. Whereas the static failure is brush-like, the fatigue failures at stresses lower than ∼60% of the static strength are slivery. The failure mode at the elevated temperature tends to be more slivery than at room temperature. The composite is susceptible to modulus reduction in fatigue without any apparent longitudinal splitting. The maximum modulus reduction before failure increases with decreasing stress level.