Traditionally, soft body armor has been made from materials such as poly(p-phenylene terephthalamide) (PPTA) and ultra-high molar mass polyethylene (UHMMPE). However, to diversify the fiber choices in the United States body armor market, copolymer fibers based on the combination of 5-amino-2-(p-aminophenyl) benzimidazole (PBIA) and PPTA were introduced. Little is known regarding the long-term stability of PBIA fibers, but as condensation polymers, they have potential sensitivity to moisture and humidity. Ballistic resistance and other critical structural properties of these fibers are predicated on their superior mechanical properties. Therefore, it is important to characterize the strength of these materials and understand their vulnerability to environmental conditions to evaluate their use lifetime in safety applications. Three PBIA-based fibers were selected for the study. The fibers were thoroughly washed to remove an organic coating, which held the individual fibers in each yarn bundle together, allowing for the disentangling of single fibers for mechanical testing. Molecular spectroscopy and single fiber tensile testing were performed on the fibers to characterize changes in their chemical structure, tensile strength, and strain to failure as a function of exposure time to four different hydrothermal ageing conditions.