A process and system for adsorption purification of NF.sub.3 wherein a
crude product containing NF.sub.3 and impurities such as CF.sub.4 is
brought into contact with a polyacrylonitrile-based carbon molecular
sieve so that at least a portion of one or more impurities are adsorbed
by the sieve without a significant adsorption of the NF.sub.3.