The oscillations of the absorbance change due to molecular vibrations in pseudoisocyanine bromide J-aggregates were real-time resolved with sub-5-fs laser pulses. In spite of weak exciton-phonon coupling in the Frenkel exciton system, it was possible to observe the modulation of difference absorbance because of the enhanced transition moment. The non-Condon-type exciton-phonon coupling was invoked to explain the oscillatory signals and the transition dipole modulations were evaluated. The band at 38 cm(-1) was assigned to optical phonon mode, and the band at 152 cm(-1), which had never been detected in Raman scattering, was also real-time resolved. (C) 2004 Elsevier B.V. All rights reserved.