Quantitative analysis of Cu K-edge XANES spectra representing mixed states of two cuprous compounds was studied to determine the composition of copper species formed in the oxidative carbonylation of phenols. The quantification of two Cu(I) species, Cu2O and an unusual cuprous dibromide complex in the linear configuration stabilized with tetrabutylammonium cation, could be achieved through detailed edge analyses to choose proper reference XANES spectra, followed by fitting the experimental XANES spectrum with the linear combination of the two reference spectra. The amount of cuprous dibromide complex obtained by this method showed an excellent correlation with the rate of formation of the desired para-position carbonylation products, indicating that the phase was the active and selective catalytic phase of copper.