The interfacial transition zone tends to be the weakest link in cement-based materials. This zone is widely believed to be less dense compared with the bulk hydrated cement matrix and is often the location where the failure first initiates. Because of its increased porosity, the interface also acts as a preferential path for transport of deleterious fluids. Understanding the microstructure of the interface, therefore, is of high importance. In this paper, a new technique is developed to calculate the width, as well as the average porosity, of the interface. The method relies on computer tomography (CT). Compared with the traditional porosity-measurement methods, the proposed dual-scan method is non-destructive, and, to a great extent, preserves the initial microstructure of the interface, thereby providing an undisturbed characterization. This article further attests that the dual-scan method can mitigate the deficiency of traditional CT scan in terms of its limited resolution.