The Diffusive Interfacial Transport (DIT) method for performing phase studies (in which all the phases that exist along an isotherm are produced by isothermal swelling and their compositions determined by using refractive index data) was introduced in 1987.(1) While this method represented a significant qualitative advance in phase studies methodology, its quantitative accuracy has since been found to be unsatisfactory. We describe herein the DIT-NIR method in which execution of the study is the same as before, but peak-area data on the bend-stretch combination band of water are used to determine composition. Peak-area data are obtained by using Fourier transform near-infrared (FT-NIR) microspectroscopy. Calibration studies with octyldimethylphosphine oxide-water mixtures demonstrated that peak areas vary linearly with composition over the entire composition range. The slopes of calibration lines follow a power-law dependence upon temperature. Phase studies of the C12E3-water system, using both DIT-NIR and isoplethal methods, validate the applicability of the DIT-NIR method to systems other than phosphine oxides. These data suggest that DIT-NIR may represent a new general method for the complete and accurate quantitative determination of binary aqueous surfactant phase diagrams. Further, the spectroscopic data can provide new information on the physical state of the material under study.