Aramid and poly( p -phenylene benzobisoxazole) (PBO) fibers are two of the most represented organic fibers possessing high strength, high modulus, excellent thermal stability, and chemical resistance, with great potential in oil and gas applications. The reliability of organic fibers for oil and gas applications were systematically evaluated by studying the corrosion behavior and mechanisms in high-temperature and high-pressure (HTHP) hydrogen sulfide (H 2 S) and carbon dioxide (CO 2 ) corrosive environments. Scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), X-ray diffraction (XRD), elemental analysis, density measurements, and single fiber tensile testing were conducted to study the surface morphology, chemical structure, crystal structure, thermal, and mechanical properties of aramid and PBO fibers, before and after corrosion. After corrosion, the crystallinities of aramid and PBO fibers decreased by 19.4 and 4.4%, respectively, whereas their tensile fracture strengths decreased by 50.34 and 28.18%, respectively. Hence, the corrosion resistance of PBO fiber is better than aramid fiber. The decrease in tensile properties of aramid and PBO fibers can be attributed to the higher internal porosity, more number of surface defects, and lower crystallinity after HTHP H 2 S/CO 2 corrosion. This work provides some fundamental information regarding the selection of high-performance organic fibers for oil and gas applications.