We have constructed a new system of narrow gap infrared reflection absorption spectroscopy (NG-IRRAS) for solid-solution interface biomaterials. A sample solution is confined in a narrow space between the CaF2 prism and the substrate surface. The sample holder is specially designed to keep the narrow space constant during the measurement process. Absorption spectra of the solution phase immunoglobulin G (IgG) and the solid-solution interface IgG covalently bonded to the carboxyl-terminated self-assembled monolayer on the gold surface have been clearly observed with good reproducibility. Noticeable molecular conformation change induced by adsorption was not observed. (C) 2008 Elsevier B. V. All rights reserved.