Direct spectroscopic evidence of the presence of organic phenolate ions at the air/aqueous interface has been established using the nonlinear spectroscopic technique of vibrational sum frequency generation (SFG). By probing the phenol C-OH and the phenolate C-O vibrational stretches, which occur in the range of 1200-1300 cm(-1) and are distinguishable, we have succeeded in determining the identities of the chemical species present at the air/aqueous interface of phenol-phenolate solutions at bulk pH values ranging from 3.2 to 13.2. We have found that in addition to phenolate ions that phenol is present at the interface at high bulk pH values. (C) 2008 Elsevier B. V. All rights reserved.