Heterogeneous (gas-solid) photochlorination reactions of a poly(4-methyl-1-pentene) film were studied with the objective of determining factors that control the surface selectivity of the modification reaction. By adjusting the chlorine vapor pressure, light source intensity, and reaction time, the depth of the chlorination reaction (thickness of the modified layer) and the extent (density) of chlorination (Cl:C ratio) could be independently controlled. Gas permeabilities of asymmetric membranes prepared by this reaction to hydrogen, carbon dioxide, oxygen, and nitrogen were determined. Light chlorination (law Cl:C ratio) at both deep and shallow levels and extensive (Cl:C similar to 1:1)/shallow chlorination yielded membranes exhibiting no changes in permeability relative to virgin poly(4-methyl-1-pentene). A densely and deeply chlorinated membrane was shown to have improved selectivities over those of unmodified poly(4-methyl-1-pentene). The selectivity increased the most (from 16.5 to 95.9) for H-2/N-2 and the least (from 2.1 to 2.6) for CO2/O-2, with accompanying decreases in flux.