Ultra-thin MFI membranes were evaluated for N-2/He separation over the temperature range of 85-260 K for the first time. The membranes were rather nitrogen selective at all the conditions investigated. A highest N-2/He selectivity of 75.7 with a high N-2 flux of 83 kg/m(2)/h was observed at 124 K. The separation was attributed to adsorption selectivity to N-2, effectively hindering the transport of He in the zeolite pores. The exceedingly high permeance even at low temperatures was ascribed to the ultrathin (<1 mu m) membrane used. As the pressure ratios increased, a better separation performance was obtained. A mathematical model showed the largest difference of adsorbed loading over the film at ca. 120 K was the main reason for the observed maximum selectivity. Further, the modelling indicated the selectivity would increase 2-3 times by reducing the influence of defects, concentration polarization, and pressure drop over the support. (C) 2016 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers