The formation of nitrogen cluster ions [N+2(N2)n,N+(N2)n> should occur easily in Triton's thin cold nitrogen atmosphere. These ions are formed stepwise as: N+2+N2→N+2(N2)+N2→N+2(N2)2 +M2→⋅⋅⋅+N2→N+2(N2)nin At low altitudes, thermodynamics (temperature and pressure) favor cluster sizes up to and including N+2(N2)40. Such ions are clos to the threshold (critical size) to nucleate, forming a stable nitrogen ice aerosol seen as extended hazes in Voyager images of Triton's limb. The critical ion radius for nucleation is about 8.3 ¿. As temperature increases and pressure decrease with altitude, cluster ion formation becomes less favorable and small cluster sizes predominate. At altitudes >200 km, cluster are thermodynamically unstable and only N+2 and N+ are present. Because electron recombination with atomic (ions (N+) is very slow, a dense ionosphere should form where N+ recombination via N+ clustering with N2 is unfavorable. Model calculations yield peak electron concentratiosns above 200 km, as detected by Voyager radio occultation measurements. The sharp ledge at the bottom of the ionosphere is caused by the onset of rapid electron-ion recombination associated with cluster ion formation. ¿The American Geophysical Union 1990