Assembling properties of novel tropoquinone derivatives which formed J-aggregates in acidic solutions were studied in monolayer assemblies. The surface pressure-molecular area isotherms showed two assembling states below and above about 30 mN m-1. Langmuir-Blodgett (LB) films of tropoquinone derivatives with propyl chains were prepared at both higher and lower surface pressures than 30 mN m-1. The absorption spectra of a single-monolayer LB film deposited at 20, 30 and 40 mN m-1 and pH 1 resembled those in n-hexane but were red shifted, strongly suggesting aggregation in the LB films. Extended dipole model calculations of chromophore interactions using the observed spectral shift indicated that about 25 and six chromophores aggregated in a two-dimensional array in acidic solutions and in LB films respectively.