To understand the mechanism of thermochromic phase transition of a J-aggregate monolayer of a merocyanine dye (MD, 3-caboxymethyl-5-[2-(3-octadecyl-benzothiazolin-2-ylidene)ethylidene]rhodanine) on aqueous subphases containing Mg2+ and Cd2+ ions, the molecular vibrational investigation was performed by using external infrared (IR) reflection absorption spectroscopy. The in situ observation indicates that, on increasing the subphase temperature from 19.5 to 34.5 degreesC, IR bands observed at 1177 and 1126 cm(-1) for the J-aggregate monolayer shift to 1188 and 1146 cm(-1), respectively; the shift is reversible and exhibits a thermal-hysteresis. These spectral changes correspond well to the thermochromic behavior of the visible absorption band of the J-aggregate (J-band), where the J-band shift is observed between 620 and 595 nm. The ab initio calculation of the normal modes were carried out on the MD molecule to assign the IR bands near 1183 and 1136 cm(-1), indicating that they are attributed by the vibrational modes of the carboxymethyl group coupled with the modes of the thioketon group of the rhodanine ring, the conjugated backbone of the dye, and the benzothiazole C Group. Both spectroscopic and computational results suggest that the transition from the phase of the J-band at 620 nm to that of the J-band at 595 nm is characterized by the exchange of the metal ion from Mg2+ to Cd2+ interacting with the MD molecule through the carboxylate group of the carboxymethyl group, and the exchange of metal ion induces the shift of the IR bands as well as the J-band.