The change (Deltamu) in the permanent dipole moment on the S,, So electronic excitation is determined for Coumarin 153 (C 153) in a molecular beam from the spectral change induced by applying a strong dc electric field up to 200 Mcm. The comparison of the observed fluorescence excitation spectra under various external fields with the corresponding simulations for a pendular-state molecule yields Deltamu = 7.1+/-0.4 D for both the syn and anti conformers of C153. Previous experimental results on Deltamu in the condensed phase, as well as the theoretical predictions reported in the literature, are discussed on the basis of the present value under an isolated condition. (C) 2003 Elsevier Science B.V. All rights reserved.