Excitation localization in a nominally 3-fold-symmetric molecule is probed using electroabsorption (Stark) spectroscopy in polymer and organic glass matrixes. The absolute value of the change in dipole moment (\Deltamu\) of this molecule, referred to as the trimer, is found to be nearly identical to that of a model compound consisting of one of its three arms within experimental error. Electronic structure calculations presented in the companion paper to this one (Liu, L. A.; Peteanu, L. A.; Yaron, D. J. J. Phys. Chem. B 2004, 108, 1684 1) reveal that the value of \Deltamu\ measured in the trimer is consistent with absorption to a state localized on a single branch of the molecule. Previously, localization on the emission time scale had been demonstrated for these particular trimers. The localization evident from the electroabsorption spectrum results from ground-state geometrical disorder in the branches that renders their excitation energies nonequivalent and reduces the overall symmetry of the molecule.