J-aggregates of tetraphenyl porphyrin tetrasulfonic acid (TPPS) doped in a polyviny-lalcohol (PVA) thin film were studied by highly sensitive electroabsorption spectroscopy using a multichannel lock-in amplifier. A change in the static polarizability Delta alpha of the aggregate upon electronic excitation was determined as 20000-39000 angstrom(3), from which the size of coherent delocalization of excitons was evaluated to be 60-80 molecules. This is ten times larger than that previously reported. At least one of the reasons of the difference is found to be the content of water molecules in the polymer matrix. (c) 2005 Elsevier B.V. All rights reserved.