Water at the asphalt/aggregate interface is the major contributor to the debonding of asphalt from mineral aggregates ( stripping). A technique based on Fourier transform infrared ( FTIR) spectroscopy in the multiple internal reflection (MIR) mode to measure in situ the water layer at the interface between an asphalt and a model siliceous aggregate is described. An asphalt layer of approximately 70 mm thick was coated to an SiO2-covered Si prism, which serves as the model siliceous substrate. A water chamber was attached to the asphalt-coated substrate. FTIR-MIR spectra were taken automatically at specified time intervals without disturbance to the specimen or the optical alignment of the instrument. The amount and thickness of the water layer at the interface between an asphalt and a siliceous substrate were determined based on a two-layer model derived from the internal reflection spectroscopy theory. The application of this technique for measuring the thickness of the water layer at the asphalt/model siliceous interface for several asphalts is presented. Based on interfacial water data, the mechanism of the stripping of an asphalt from a siliceous aggregate and the transport processes of water from the environment to the asphalt/aggregate interface are discussed.