The monolayer assemblies incorporating two kinds of unsymmetric merocyanine dye aggregates (MC(X)/MC(Y), X = O, S. Se) have been fabricated by the Langmuir-Blodgett technique. The absorption spectra of the Langmuir-Blodgett films were measured in wide temperature range of T = 6.0-293 K. The absorption spectra of the MC(O)/MC(S) and the MC(O)/MC(Se) assemblies bear two absorption bands which are assigned to the MC(O) aggregate and the MC(S) J-aggregate or the MC(Se) J-aggregate, respectively. In contrast, the absorption spectrum of the MC(S)/MC(Se) assembly shows a single absorption band of the mixed J-aggregate. From detailed considerations of the low-energy tail of absorption spectra based on the Urbach-Martienssen rule, together with other spectroscopic properties, it is concluded that the MC(O)/MC(S) or the MC(O)/MC(Se) assembly forms a mosaic aggregate. The MC(S)/MC(Se) assembly can form high-quality homogeneous aggregate of amalgamation type. (c) 2006 Elsevier B.V. All rights reserved.