Oxidized groups on the outer surface of HNO3-treated carbon nanofibers (CNFs) have been quantified using 1-naphthol (1-NP) as an in-situ fluorescent probe, whereas the characterization was difficult by IR spectroscopy. The fluorescence measurement allowed us to confirm the Langmuir-type adsorption of 1-NP on the graphene sheet and the oxidized groups of the CNFs, and estimate the amounts of both sites and their adsorption equilibrium constants. The number of oxidized sites per nm(2) on the CNFs treated for 1 and 24 h were estimated to be 0.46 and 1. 18, respectively. (c) 2005 Elsevier B.V. All rights reserved.