Surface infrared spectroscopy was carried out on unreconstructed H:Si(111)1 x 1 and D:Si(lll)1 x 1 and the spectra obtained were compared precisely. Two narrow absorption peaks of Si-PI bond-stretching and bond-bending were clearly observed on H:Si(111), while on D:Si(111) a narrow peak for the Si-D stretching was observed. But the corresponding bending peak split into two and these widths were from three to four times larger than that of Si-H. The splitting of the bending vibration is direct evidence of the coupling of the vibration of the surface layer composed of D and Si with the second layer of Si. Isotope exchange from H to D leads to the decrease in bending vibration frequencies down to those of the substrate. The broadening suggests the interaction of the deuterized surface layer with the rest of the substrate.