In the previous papers, the author and others reported on the correlation of spectral sensitization by merocyanine dyes with their adsorption in zinc oxide electrophtographic layers, and concluded that the sensitization occurred only when a polar (particularly an anionic or an acidic) group was introduced into the dye structure. In the present paper, the adsorption state of an acid morocyanine dye having a carboxymethyl groups studied by means of infrared absorption spectroscopy. The application of infrared spectroscopy to the study of zinc oxide-dye system has been found possible, since zinc oxide has weak absorption in the range of 3, 000-1, 100 cm^-1. The acid merocyanine dye has an absorption band at 1, 720 cm^-1 when mixed with KBr or ZnO, but loses the band when adsorbed onto ZnO. This fact means the chemical change of the carboxylic acid group to carboxylate (zinc salt) due to the adsorption. In other words, the acid merocyanine dye is chemisorbed onto ZnO. Experimental results obtained in stearic acid-ZnO system support the idea. The fact that almost all the excellent sensitizing dyes for ZnO-electrophotography are acid ones suggest the chemical interactions between ZnO and dyes.