We have investigated the influence of heat treatment in an air atmosphere (HT) and hydrothermal treatment in the liquid phase (HTTL) on the H-aggregate in a mixed Langmuir-Blodgett (LB) film of merocyanine dye with an octadecyl group (MS 18)-arachidic acid (C:20)-n-octadecane (AL(18)) ternary system by means of polarized visible and IR absorption spectroscopy. HT causes the variation from the H-aggregate to the monomer, the increment in the number of gauche conformers in the MS18 hydrocarbon chain, the slight orientation change in the C-20 hydrocarbon chain, and the complete evaporation of AL(18). The dissociation of MS18 is probably ascribed to the complete evaporation of AL(18) from the mixed LB film and the increase in thermal mobility of the long axis of the MS18 hydrocarbon chain during HT. However, HTTL can easily and rapidly induce the conversion of the MS18 aggregation state from H- to J-aggregates, the modification of the C-20 subcell packing from hexagonal to orthorhombic, and the removal of most of the AL(18) molecules. The conversion of the MS18 aggregation state can be interpreted to consist of two processes from the H-aggregate to the monomer and from the monomer to the J-aggregate. In the initial stage of HTTL, the MS18 aggregation state changes from the H-aggregate to the monomer, which is caused by the removal of almost all of the AL(18) molecules from the mixed LB film to warm water via the thermal energy of warm water. Then, the large relative permittivity of warm water is expected to relate strongly to the subsequent variation from the monomer to the J-aggregate. This transformation results in the decrease in the total value of the electrostatic energy based on the MS18 permanent dipole interaction. Moreover, the modification of the C-20 subcell packing is possibly due to the hydrophobic effect, where the C-20 hydrocarbon chains cohere again in the warm water during HTTL. Consequently, it has been found that HTTI. is quite effective to reorganize the chromophore alignment of MS18, to modify the subcell packing of C-20 and to erase the majority of AL(18) molecules in the mixed LB film of the MS18-C-20-AL(18) ternary system in a short time.