The permeation and separation performance of an ultramicroporous carbon membrane for separation of CO2/N2 mixtures were investigated. The experiments were conducted using the steady-state measurement method with pure gases (dead-end mode) and a CO2/N2 gas mixture (20/80?mol.-%) (cross-flow mode) in the temperature range from 293?K to 363?K and at feed pressures of up to 1.4?MPa and atmospheric pressure on the permeate side. The membrane exhibited a selectivity of about 25 and permeability of about 500?Barrer for CO2 in the mixture with N2. The single-gas measurements do not reflect the membrane performance correctly. An adsorption-selective effect is assumed to be the main separation mechanism. Moreover, membrane-aging effects causing blocking due to pore constrictions through adsorption were observed. These pore constrictions lower the permeability, but they raise the selectivity. Operation at high temperatures leads to a reduction of aging effects.