Abstract: Gases having molecular dimensions equal to or smaller than nitrogen are selectively adsorbed and separated from other gases having molecular dimensions equal to or larger than methane using a clinoptilolite molecular sieve that contains magnesium cations. The clinoptilolite molecular sieves of the present invention can be ion-exchanged with magnesium cations and optionally with other cations such as lithium, sodium, potassium, calcium, barium, strontium, zinc, copper, cobalt, iron and manganese to produce the desired separation characteristics. Processes are disclosed for separating gases by adsorbing at an elevated pressure and regenerating, or desorbing, by reducing the pressure. Methods for the preparation of a magnesium-exchanged clinoptilolite are disclosed.