The textbook view of the liquid electrolyte surfaces as being devoid of ions have recently been challenged by molecular dynamics simulations, which predict a surface enhancement of highly polarizable anions. Here we present the first direct experimental verification of this prediction. Enhanced azide (N-3(-)) concentrations were measured at the liquid surface by femtosecond second harmonic generation (SHG) experiments exploiting the charge-transfer-to-solvent (CTTS) resonance of N-3(-), yielding a surface excess free energy of -9.9+/-0.3 kJ/mole. Such surface-enhanced concentrations of anions could have important consequences for the chemical reactions taking place on atmospheric aerosols and at the ocean-air interface. (C) 2004 Elsevier B.V. All rights reserved.