This work reports on the results of investigations conducted with three layered polypropylene (PP) membranes which were exposed to a N(2) flowing microwave (100 W) post discharge, at several flow rates (0.03-1 Slm) and at gas pressure from 0.3 to 40 Torr. The N atoms transmission through the membrane was measured from the intensity variation of the N(2), 580 nm afterglow which is related to N atom density, before and after the membranes. The results obtained showed that N-atoms penetrate and cross the membranes with a transmission factor (T (N)) varying from 20 to 70% during the exposure time (15 min), depending on flow rate and gas pressure. A maximum value T (N) = 0.7 (+/- 0.05) is found for sufficient low flow rate (less than 0.1 Slm), that is for low flow gas velocity (about 2 x 10(2) cm s(-1)).