Disclosed are novel, as-crystallized RHO zeolite compositions with improved uniformity in morphology. These RHO compositions are useful for preparing cation-exchanged RHO zeolites that can be used as adsorbents for oxygen and/or nitrogen with improved properties. The adsorbents can be used in pressure swing adsorption processes for selectively adsorbing oxygen and/or nitrogen from feed streams such as an air stream or crude argon stream. Also disclosed are novel methods of preparing RHO zeolites with improved morphology control.