Pretreatment stage is usually a requirement for any adsorption based air separation process. Carbon dioxide and water vapor present in the act as contaminants, deactivating adsorbents, particularly zeolites used in oxygen pressure swing adsorption processes. Such systems usually present one or more prelayers to ensure full removal of these two contaminants, protecting the oxygen/nitrogen selective layer. In the present study, two 13X-type zeolites one activated alumina and one highly pure silica are compared in terms of capacity for water vapor and carbon dioxide removal from air. Water and carbon dioxide adsorb irreversibly on these adsorbents up to a certain extension and then effective adsorption isotherms and breakthroughs curves were obtained. The effective properties were attained after three cycles under close to vacuum pressure swing adsorption conditions. A combination of two layers for the precolumns is suggested: the first, composed by either silica or alumina to remove most of the water without significant loss of cyclic adsorption capacity, and a second, composed by zeolite, to reduce the amount of water and carbon dioxide down to parts per million (ppm) levels. These should prevent contamination and consequent loss of efficiency in the nitrogen/oxygen selective layer.