Non-common-atom (NCA) heterovalent interfaces, offer potential benefits arising from the valence mismatch but also contain challenges due to the large charge imbalances. Whether a sharp polar interface is formed or, on the contrary, there are mixtures of chemical bonds across the interface leading to a nonpolar graded interface is under intense discussion. Furthermore, very little has so far been done to exploit the opportunities offered by NCA heterovalent interfaces, in part due to challenges determining the structure and properties of these types of interfaces, for example, by using scanning transmission electron microscopy (STEM) techniques. This work presents a comprehensive analysis of the composition profile and strain across the case study NCA heterovalent CdTe/InSb interface, carried out using a combination of (S)TEM imaging and spectroscopic techniques. Techniques such as high-angle annular-dark-field and large-angle bright-field scanning (S)TEM, as well as electron energy-loss spectroscopy, give results from the interface region on the atomic scale. These measurements, however, are inherently difficult to interpret because of the close atomic numbers of the constituent elements. In contrast, use of the 002 dark-field TEM imaging mode emphasizes the interface location by comparing differences in structure factors between the two materials. Based on the 002 dark-field TEM, a methodology was developed for reliable determination of the composition profile across the interface and systematic quantification of the interface width. Since the identities of both cations and anions change across the heterointerface, the respective contributions of different elements need to be inserted separately. The intermixing at each sub-lattice is thus independently and completely determined. Comparisons of experimental and simulated CdTe-on-InSb profiles reveal that the interface is structurally abrupt to within about 1.5 nm defined by the variation between 10 and 90%, while local strain measurements with geometric phase analysis based on aberration-corrected high-resolution STEM micrographs reveals a minimal level of interfacial strain. The present investigation opens new routes to the systematic investigation of heterovalent interfaces, formed by the combination of other valence-mismatched material systems.