Kinetic evidence is presented for the formation of nitrogen-atom islands on Rh(111) surfaces. The deposition of atomic nitrogen on that surface was carried out via the steady-state conversion of NO with CO by using effusive collimated beams. Isotopic labeling experiments were performed where N-14-dosed surfaces were subsequently exposed to (NO)-N-15 + CO gas mixtures for varying times. Subsequent temperature-programmed desorption indicated a nonstatistical distribution of isotopes in the resulting molecular nitrogen, the yield of the mixed (NN)-N-14-N-15 isotopomer being significantly lower than that expected on statistical grounds. Monte Carlo simulations were used to explain the observed isotopic distributions in terms of the formation of islands with the nitrogen isotopes distributed in a layered structure, the N-14 atoms in a core surrounded by a N-15 outer shell.