The scope and limitations of photooxidative surface modification of polyimide films were investigated as a means of improving the selectivities of air separation membranes without drastically reducing their permeabilities. Membranes of polyimides that contained the phthalimide chromophore and abstractable hydrogens showed significantly higher oxygen/nitrogen selectivities after being irradiated with approximately 200-300 nm light in the presence of oxygen for 0.5-30 min. Not only were benzylic hydrogens found to be abstractable, but also certain t-butyl hydrogens could be abstractable under these conditions. A mechanism for the photochemical reaction that is based on the photochemistry of structurally similar monomeric phthalimides is proposed to explain the observed selectivity enhancement.