Nanosecond time-resolved infrared spectroscopy has been used to study the excited electronic states of fluorenone in polar and nonpolar solvents. By using the singular-value-decomposition analysis of the observed time-resolved spectra, infrared spectra of fluorenone in the lowest excited singlet (S-1) and the lowest excited triplet (T-1) states have been obtained for a polar solvent, as well as that in the T-1 state in a nonpolar solvent. The assignments of the CO stretching modes for the S-0, S-1 and T-1 states have been made on the basis of isotopic frequency shifts. The CO stretch frequency of fluorenone in the S-1 state in a polar solvent and those in the T-1 states in polar and nonpolar solvents are much lower than that of the ground state, but are still located in the double-bond frequency region. These findings indicate that these excited states are the same in character (pipi*), but differ in the degree of electron delocalization between the benzene rings and the CO bond. (C) 2003 Elsevier B.V. All rights reserved.