Molecular aggregation of an oligo(2,5-dialkoxy-1,4-phenyleneethynylene) 4 was successfully detected by simply using a combination of a proper solvent/nonsolvent system with low temperature. The study evaluated aggregation solely originating from intermolecular chromophore-chromophore interaction under high dilution. Spectroscopic investigation showed that the aggregate absorption band of 4 in solution was red-shifted by similar to29 nm from its 0-0 absorption band (lambda(max) approximate to 431 nm). In the aggregate state, the chromophore molecules appeared to be arranged in a parallel fashion for a maximum level of interaction. Absorption and fluorescence spectra of film 4 showed that the aggregate structures formed in its solution were largely retained in the film state. The study provided useful information to aid the understanding of aggregation of polymer 1, which possessed the effective chromophore of the same conjugation length, and allowed both inter- and intramolecular chromophore-chromophore interactions.