A long-distance chromatographic operation was carried out in order to study the isotope accumulation of N-15 in the ion-exchange enrichment process by using a porous microreticular cation-exchange resin. Combined bands of ammonium and lithium ions were eluted up to 20 m, and thereafter the ammonium band was eluted up to a total migration distance of 60 m. The constant lengths of the adsorbed bands were maintained throughout the elution, and a sharp boundary at the rear end of the ammonium band was monitored by an electric conductivity meter. The enrichment of both N-15 at the rear end of the ammonium band and Li-6 at the rear end of the lithium band were confirmed in the experiment. The values of HETP in the system were kept almost constant, 0.49 mm, throughout the migration up to a distance of 60 m. Previously reported works on the same process are analyzed, and an empirical relation for HETP in terms of resin diameter and band velocity has been obtained as HETP = c(d(p)u(B))(0.84).