A novel method to study competitive molecular interactions in a multicomponent system has been introduced. It is a method based on the framework of a two-dimensional infrared (2D IR) correlation spectroscopy technique with a new data pretreatment strategy, namely, the component-normalization method. In this method, the absorbance of each absorption band is normalized with the concentration of the corresponding component in the system. Competitive interactions are expressed quantitatively by a normalized 2D IR parameter, which is a correlation coefficient of the synchronous 2D IR spectrum. Using the newly proposed method, we examined molecular interactions among dimethyl sulfoxide (DMSO), n-decylamine, and water. It was found that the interaction between DMSO and water is the greatest among the three interaction pairs. Furthermore, it was demonstrated that, by a systematic perturbation in the concentration of DMSO of the three-component mixtures, the sequential order of response by individual functional groups in the system could be traced. Thus, the S=O group (DMSO) is affected first, followed by the hydroxyl group (water) and, finally the amine group (n-decylamine) as DMSO is added to the system. The results have been used to elucidate the possible working mechanism of the dehydration effect of DMSO on hydrated biosystems.