Electron paramagnetic resonance (EPR) studies of low hydroxyl synthetic silica optical fibers have revealed the presence of a 13 G hyperfine splitting of the SiE’center which is associated with weakly bonded hydrogen molecules in the silica structure. The fibers were previously exposed to hydrogen at room temperature. During an annealing of the silica fibers at 100 °C in air, the 13 G doublet was observed to decline and disappear while the 74 G doublet and the SiE’signal were growing. The result correlates with the redistribution of hydrogen molecules and is relevant to the environmental aging that is encountered in fiber optical waveguides under certain conditions. ©1995 American Institute of Physics.