In the present work the performance of two zeolite sorbents, namely LiX (Al/Si = 1 with 100% Li+) and NaX (13X), is compared for air separation by pressure swing adsorption, with particular attention focused on the limits in the adsorption/desorption pressure ratio. The product (O-2) recovery was optimized far the two sorbents at different pressure ratios (2-10), keeping the the product purity and product throughput nearly constant. It was found that in the case of the LiX zeolite it is possible to operate at a pressure ratio as low as 2 with only a slight decrease in product recovery, compared to the limit of 4 for NaX. The temperature excursion in the LiX bed was 4 times that in the NaX bed. Substitution of a portion of the LiX sorbent by inert, high-heat-capacity particles (e.g., iron) in the bed decreased the temperature excursion significantly. It was shown that an optimal amount of 5-10% (v/v) of inert substitution (at the same total bed volume) could increase the O-2 product recovery by 2%.