Absorption and emission spectra of a monolayer J aggregate of a cyanine dye have been studied in detail from room temperature to liquid helium temperature. Various sources of disorder that affects the degree of delocalization of the exciton have been investigated including the hitherto unnoticed effect of He gas. Simple numerical simulations were performed, which reproduce the basic features of the spectra fairly well. The spectral signatures indicate that we have a typical two-dimensional Frenkel exciton system characterized by large red shift and large cooperation number despite strong disorder, which is in contrast to the well-studied pseudoisocyanines ͑PICs͒ that have been analyzed as one-dimensional even in the Langmuir-Blodgett monolayers. The disorder stems partly from the exciton-phonon coupling but is dominantly of the static character at low temperatures.

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