A new thermochromic transition was observed in merocyanine dye (MD)/arachidic acid mixed monolayers with the subphase which contained cadmium and magnesium ions. Special cells were fabricated to observe the phenomenon. By heating the cell above 27.8 degreesC, the J-band at 620 nm changed to 595 nm irreversibly with an isosbectic point around 600 nm.