We present here a comparative planetology study of evolution of N-14/N-15 at Mars and Titan. Studies show that N-14/N-15 can evolve a great deal as a result of escape in the atmosphere of Mars, but not in Titan's atmosphere. We explain this through the existence of an upper limit to the amount of fractionation allowed to occur due to escape that is a function of the escape flux and the column density of nitrogen. (C) 2015 Elsevier Inc. All rights reserved.