Abstract: Samples of mixture of benzoic acid and potassium hydrogen phthalate were determined by Fourier transformation midinfrared spectroscopy (FT-MIR), and the IR spectra obtained were transformed into its second derivative spectra. Using the above 2 spectra as correction set samples, the spectral data were optimized and condensed by the method of principal component analysis to establish the model for quantitative analysis. The proposed method was used in the determination of mixture of benzoic acid and potassium hydrogen phthalate, giving predictive values in consistency with the substantial values.