Here, we describe the formation of zeolite LiX fiber sorbents and postfabrication activation at temperatures of 350-400 degrees C for use in air separations. Two polymers were considered as the support matrices for the LiX zeolites: Torlon (polyamide-imide) and Matrimid (polyimide). Torlon-based fiber sorbents exhibit approximately 4% mass loss during the high-temperature (370 degrees C) activation required for zeolites, which is close to the glass transition temperature of these polymers. This mass loss is accompanied by the formation of pyrolytic side products that ultimately resulted in contamination of the LiX crystals and a 73% reduction in N-2 sorption capacity at ambient temperatures. In contrast, Matrimid was found to be suitable for fiber sorbent synthesis with only 10-15% reduction in N-2 sorption capacity observed upon the thermal activation. A LiX loading of 73% in monolithic fibers (i.e., no bore) by weight was achievable. Experimental breakthrough tests at 25 degrees C were conducted on an activated fiber module consisting of densely packed LiX-Matrimid fiber sorbents and operated over a range of adsorption pressures (1-3.5 bar) that are often used in an actual pressure swing adsorption (PSA) process for air separation.