J-aggregates in Langmuir-Blodgett (LB) films of mutually mixed merocyanine dyes were characterized by using Fourier transform infrared (FT-IR) spectroscopy. Two merocyanine dyes, one which forms and one which does not form J-aggregates in their LB films with fatty acids and divalent metals, so-called 6MeDS and DO, respectively, were used in this study. In LB films of DO-6MeDS mutual mixtures, most DO molecules are included in J-aggregates. The IR spectra of these films were measured and compared with linear combinations of the spectra for DO and 6MeDS films. The difference between the measured and the calculated IR spectra reflects the difference between the electronic state of J-aggregated DO and that of monomeric DO. The results suggest an enhancement of the intramolecular charge transfer induced by the formation of J-aggregates.