NOVELTY - Separating at least one gas from a mixture of gases comprises: providing a chemically crosslinked polymer membrane comprising an organic polymer, organosilsesquioxane segment and covalent bond between the organic polymer and organosilsesquioxane segment; contacting the mixture of gases to a first side of the membrane to cause the gas to permeate the crosslinked organic-inorganic hybrid membrane; and removing from a second side of the membrane a permeate gas composition comprising at least a portion of the gas which is permeated the crosslinked organic-inorganic hybrid membrane.    USE - The method is useful for separating: at least one gas from a mixture of gases comprising a pair of gases, which are carbon dioxide/natural gas, hydrogen/methane, carbon dioxide/nitrogen, methane/nitrogen, iso/normal paraffins, olefins/paraffins (all claimed), oxygen/nitrogen or polar molecules including water, hydrogen sulfide and ammonia/mixtures with methane, nitrogen, hydrogen and other light gases; and variety of liquids.    ADVANTAGE - In the chemically crosslinked polymer membrane, the degree of crosslinking can be controlled easily by adjusting the molar ratio of the precursor organic polymer to the organosilicon alkoxide crosslinking agent. The chemically crosslinked polymer membranes: do not have phase separation between the organic polymer and the inorganic crosslinking agent; do not have inorganic particle size issues; have much better chemical and mechanical stability compared to inorganic filler-polymer mixed matrix membranes; exhibit extremely high plasticization resistance to condensable gases, which include carbon dioxide; are preferably durable, resistant to high temperatures and resistant to exposure to liquids; and provide gas separation properties which rival or exceed those of carbon fiber membranes or zeolite membranes. The precursor organic polymer provides a variety of properties for membrane separations including low cost, high selectivity and easy processability.    DETAILED DESCRIPTION - Separating at least one gas from a mixture of gases comprises: providing a chemically crosslinked polymer membrane comprising an organic polymer, organosilsesquioxane segment and covalent bond between the organic polymer and organosilsesquioxane segment, where the crosslinked organic-inorganic hybrid membrane is permeable to the gas; contacting the mixture of gases to a first side of the membrane to cause the gas to permeate the crosslinked organic-inorganic hybrid membrane; and removing from a second side of the crosslinked organic inorganic hybrid membrane a permeate gas composition comprising at least a portion of the gas which is permeated the crosslinked organic-inorganic hybrid membrane. An INDEPENDENT CLAIM is also included for the chemically crosslinked polymer membrane, where the chemically crosslinked polymer membrane is permeable to at least one gas.