We present the fourth-order Raman spectrum of an organic adsorbate on a solid surface. TiO2( 110) surfaces covered by p-nitrobenzoate (pNB) and trimethyl acetate were observed. In addition to phonon modes of TiO2, a weak band at 572 cm (1) was observed for the surface covered by pNB, and assigned as a molecular vibration. The coverage of pNB was estimated to be 13% using XPS. The feasibility of fourth-order Raman spectroscopy is demonstrated in observing organic adsorbates of submonolayer quantity. (c) 2008 Elsevier B. V. All rights reserved.