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The operation of adsorbers which purify ammonia synthesis make-up gas was assessed theoretically and empirically. A mathematical model predicts that nitrogen adsorption in 13X molecular sieve decreases the effluent flow rate and effluent molecular wight, and increases the effluent temperature at the start of the adsorption step. These interrelated, transient, process changes can greatly disrupt downstream unit operations. Data and observations from commercial ammonia synthesis gas adsorbers agree well with the effects predicted by the model. The effluent temperature rise can be used to monitor the condition of the adsorbent, the effectiveness of the previous thermal regeneration step, and flow distribution in the adsorber. A combination of activated alumina followed by 13X molecular sieve should reduce the transient process changes due to nitrogen adsorption, increase the capacity of the adsorber, and provide greater operating flexibility relative to 13X molecular sieve as the sole adsorbent.