The high-temperature interaction of the noble gas neon with carbon molecular sieve fibers (CMSF) was studied using temperature-programmed desorption mass spectrometry in a unique supersonic molecular beam sampling mode. Monitoring the dependence of the Ne desorption temperature peak on the effective dimension of the adsorption sites of the CMSF provides an insight into the interaction of the "nonadsorbable" neon atom with the carbon ultramicropores. Comparing the neon results with our recent data obtained for helium on the same CMSF reveals a molecular sieving effect with high selectivity between He and Ne. From a classical standpoint of pore and atomic size, it is shown that the effective pore size of the CMSF could be controlled to such extent that one enables the pore dimensions to meet those of the atom species, consequently providing the observation of He/Ne atomic sieving. The interaction of both He and Ne with the CMSF is found to be extremely strong, exceeding by up to 2 orders of magnitude that with other carbon materials of regular structure (i.e., graphite and carbon nanotubes). Two alternative mechanisms that may govern such strong interactions are suggested. One is the possible occurrence of a process in which "flexible" CMSF constrictions are dilated, thus forming an enhanced traplike confinement effect on the adsorbent species, or alternatively, the determining factor could simply be the binding with the dangling bonds' enriched restrictions.