Liquid crystalline triheptyl cellulose (THC) was synthesized directly from cellulose acetate and heptyl bromide with dimethyl sulfoxide as a single-component solvent. Synthetic conditions were studied with respect to reaction temperature, time, and yield. The molecular structure and liquid crystalline character of the THC obtained as sticky, soft, and birefringent solids at room temperature were investigated by IR, NMR, elemental analysis, and a polarizing microscope. The composite membranes consisting of a THC/ethyl cellulose blend as a dense thin selective layer and polyethersulfone as a porous thick support layer were fabricated and their air-separation performances were studied by a constant pressure-variable volume method. The data show that the air-separation performances increase dramatically with operating parameters such as pressure and temperature. No apparent dependence of air-separation ability on operating time and THC/ethyl cellulose ratio was observed.