Soft body armor is typically comprised of materials such as aramid. Recently, copolymer fibers based on the combination of 5-amino-2-(p-aminophenyl) benzimidazole (PBIA) and PPTA were introduced to the body armor marketplace. The long-term stability of these copolymer fibers have not been the subject of much research, however they may be sensitive to hydrolysis due to elevated humidity because they are condensation polymers. Efforts to evaluate the impact of environmental conditions on fiber strength is very important for the adoption of these materials in armor systems. Three PBIA-based fibers were selected for the study, and were aged at 25 °C, 75% RH; 43 °C, 41% RH; 55 °C, 60% RH; and 70 °C, 76% RH for up to 524 days. Molecular spectroscopy, scanning electron microscopy, and single fiber tensile testing were performed to characterize changes in their chemical structure, tensile strength, and failure strain as a function of exposure time to different conditions. The fibers were all found to have some reduction in strength at high humidity conditions, with an approximately 14% reduction for the copolymers and a 29% reduction for the homopolymer. Molecular spectroscopy revealed some changes which suggest that hydrolysis of the benzimidazole ring is occurring at these elevated temperatures, possibly explaining the observed change in strength.