Nitrate ions have been observed by resonant second harmonic generation in the air/water interface of sodium nitrate solutions when the nitrate mole fraction exceeds similar to 0.02 at ambient conditions. The surface propensity of nitrate is weak compared to those of other small anions determined by the same methodology, in qualitative agreement with recent simulations, experiments, and the position of nitrate in the Hofmeister anion series. The presence of nitrate ions in the interface of aqueous solutions has potential implications for chemical processes occurring in both natural and anthropogenic aqueous systems. (C) 2007 Elsevier B.V. All rights reserved.