It is a challenge to identify each phase in a multi-component polymer system and uniquely determine the interfacial properties between the different phases. Using atomic force microscopy nanomechanical mapping (AFM-NM) and AFM-based infrared spectroscopy (AFM-IR), we identify each phase, visualize structural developments, and determine the interfacial properties in a blend of three polymers: high-density polyethylene (HDPE), polyamide (PA6) and poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS). Each phase can be identified from the Young’s modulus, along with the structural development within the phases before and after compatibilization. The interfacial widths between HDPE/PA6, HDPE/SEBS and SEBS/PA6 were determined independently in one measurement from a Young’s modulus map. The structural, mechanical property development and identity of the phases were determined by AFM-NM, while AFM-IR, providing complementary chemical information, identified interfacial reactions, showed the chemical affinity of a compatibilizer with the component phases, and mapped the distribution of the compatibilizer in the ternary polymer blends. The chemical, structural and interfacial information obtained by these measurements provide information that is essential for producing mechanically robust materials from incompatible mixtures of polymers.