The permeation properties of DDR zeolite membranes in the separation of mixed gases are strongly affected by the gas species present in the mixture, their concentrations, and the operating conditions used. In this work, we investigated the mass transport properties of a DDR membrane for CO2 separation in the temperature range 25-75 degrees C up to 700 kPa. The influence of CO2 on the permeation of the other mixed gases as well as how the latter affects CO2 permeation were systematically investigated. The low temperature significantly emphasized the difference between single and mixed gas permeation, reflected in higher CO2 selectivity. In particular, the highest selectivity values were 28 and 17 when CO2 was mixed with N-2 and H-2, respectively, whereas the CO2 permeance exceeded 1.5 times the single gas value (i.e., 18 nmol m(-2) Pa-1 s(-1)) for a 40:60 CO2/H-2 mixture, being about 29 nmol m(-2) Pa-1 s(-1). The CO2 concentration in mixed gas significantly affected both permeance and selectivity, which was up to 5 times more in the presence of H-2 with respect to that with a single gas.