This paper reports the pH-induced structural changes in the surface immobilized poly(L-lysine)(PLL)film.Two-dimensional(2D) correlation analysis was applied to the Fourier transform infrared(FTIR)spectra of the surface-immobilized PLL film to examine the spectral changes induced by the alternations of the protonation state of the amino group in the side chain.Significant spectral changes in the FTIR spectra of the PLL film were observed between pH 7 and 8.The decrease in the protonation state of the amino group in the side chain induced spectral changes in the amino group as well as conformational changes in the alky]group in the side chain.From pH 1-8,the spectral changes in the amino and alkyl groups in the side chain occurred before those of the amide group in the main chain of the surface immobilized PLL film. This paper reports the pH-induced structural changes in the surface immobilized poly (L-lysine) (PLL) film. Two-dimensional (2D) correlation analysis was applied to the Fourier transform infrared (FTIR) spectra of the surface- immobilized PLL film to examine the spectral changes induced by the alternations of the protonation state of the amino group in the side chain. Significant spectral changes in the FTIR spectra of the PLL film were observed between pH 7 and 8. The decrease in the protonation state of the amino group in the side chain induced spectral changes in the amino group as well as conformational changes in the alky] group in the side chain. From pH 1-8, the spectral changes in the amino and alkyl groups in the side chain occurred before those of the amide group in the main chain of the surface immobilized PLL film.