The backfill cycle in the pressure swing adsorption (PSA) separation of air to produce an enriched oxygen product using a zeolite 5A molecular sieve was studied theoretically and experimentally. The effects of the backfill cycle, cycle configuration, backfill rate, pressurisation rate, and product rate were studied. The theory agreed well with the experimental results in predicting the product oxygen concentration over a wide range of backfill pressures, thus giving a basic understanding of the bed dynamics of the backfill cycle. The theory showed that the backfill rate and adsorption capacity of the adsorbent were the most important parameters governing the efficiency of the PSA process. We showed that, given an adequate backfill pressure, a separate pressurization step could be omitted by incorporating it with the product release step with no detrimental effect on the product oxygen concentrations while increasing adsorbent productivity.