Infrared reflection absorption (IRA) and fluorescence spectroscopies were applied to elucidate photodimerization processes induced by irradiation at 340 nm of stilbazolium cations (C18S) incorporated in Langmuir-Blodgett (LB) films of fully deuterated arachidic acid (DA) on the silver and GaAs substrates. A spacer layer consisting of monomolecular LB films of DA was inserted between the substrates and the mixed LB monolayers (C18S : DA = 1:4). Both the rate and the extent of photodimerization increased with the thickness of the spacer layer (d). The quantum yield of excimer fluorescence also increased with d. Analyses of these results confirmed that photodimerization proceeded through an excimer of C18S and that the reduction in the dimerization rate observed for the samples with smaller numbers of spacer layers was mainly due to an increase in the rate of nonradiative energy transfer to the substrates. The energy transfer on the GaAs substrate depended on d(-3), which conformed to a well-known classical theory based on the standard Forster-type energy transfer. On the other hand, the energy transfer rate on the silver substrate was proportional to d(-1), which did not agree with the classical theory. Change in the intensity of an excimer fluorescence band near 490 nm was observed from 15 monolayers of a mixed LB film (C18S:DA = 1:4) on a silver substrate as a function of t in the temperature range 77-320 K. Above 300 K, the intensity decreased precipitously with t, indicating the advance of photodimerization; the activation energy of the process was determined to be 28.4 kJ/mol. : DA = 1:4). Both the rate and the extent of photodimerization increased with the thickness of the spacer layer (d). The quantum yield of excimer fluorescence also increased with d. Analyses of these results confirmed that photodimerization proceeded through an excimer of C18S and that the reduction in the dimerization rate observed for the samples with smaller numbers of spacer layers was mainly due to an increase in the rate of nonradiative energy transfer to the substrates. The energy transfer on the GaAs substrate depended on d(-3), which conformed to a well-known classical theory based on the standard Forster-type energy transfer. On the other hand, the energy transfer rate on the silver substrate was proportional to d(-1), which did not agree with the classical theory. Change in the intensity of an excimer fluorescence band near 490 nm was observed from 15 monolayers of a mixed LB film (C18S:DA = 1:4) on a silver substrate as a function of t in the temperature range 77-320 K. Above 300 K, the intensity decreased precipitously with t, indicating the advance of photodimerization; the activation energy of the process was determined to be 28.4 kJ/mol.