This article presents an experimental investigation into the effects of temperature and heating time on the tensile strength and failure mechanisms of glass fibers. The loss in strength of two glass fiber types (E-glass and Advantex(A (R)), a boron-free version of E-glass) was investigated at temperatures up to 650 A degrees C and heating times up to 2 h. The tensile properties were measured by fiber bundle testing, and the maximum strength was found to be temperature and time dependent. The higher softening point of the Advantex(A (R)) fibers is reflected in superior high-temperature performance. A phenomenological model is presented for calculating the residual strength of glass fiber bundles as functions of temperature and time. The strength reduction mechanism was determined by single-fiber testing. Fracture mirror sizes on the E-glass fibers were related to the fiber strength after high-temperature treatment. Based on fracture mirror measurements, it was established that (1) the mirror constant of the glass, which reflects the network structure, does not change during heat treatment and (2) the strength degradation is a result of larger surface flaws present after heat treatment.