A method of treating fibers to improve resistance to high temperatures and relative humidity includes providing poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibers, exposing the PBO fibers to an atmosphere of supercritical CO2 saturated with an epoxy silane, i.e. 3-glycidoxypropyl methyldimethoxysilane. The PBO fibers are subjected to a first pressurizing and heating stage at a first temperature (50° C.) for a first period of time. The PBO fibers are then subjected to a second pressurizing and heating stage including increasing the temperature to a second temperature (110° C.) and holding at the second temperature for an additional period of time. The PBO fibers include 2-5 wt % of 3-glycidoxypropyl methyldimethoxysilane after diffusing the CO2 out of the PBO fibers. The fibers are particularly useful in making lightweight body armor system components.