Abstract Lightweight concrete (LWC) and normal weight concrete (NWC) are used in combination in structural engineering practice. Introduction of LWC brings advantages in terms of energy efficiency and reduction of costs. The mechanical properties of the interfacial transition zone (ITZ) between LWC and NWC has a crucial effect on the durability of structures. In this study, the bond properties of normal weight concrete-to-lightweight concrete (NWC-to-LWC) and lightweight aggregates-to-cement paste (LWA-to-CP) have been investigated at the microscale conducting micro-hardness tests and scanning electron microscope observations, whereas phase composition at the ITZ was measured with micro-Raman spectroscopy. The results indicated that the micro-hardness profile across the NWC-to-LWC ITZ is “U-shaped” and is surrounded by many fine cracks. The minimum in micro-hardness corresponds to a higher concentration of hydration products and ettringite. The LWA-to-CP ITZ is narrower; it appears compact with an asymmetric micro-hardness profile, thicker on the side of the cement paste (about 50 μm). On this side, a weak zone, with hardness lower than the one of the cement paste and enriched in ettringite and carbonated Ca(OH)2, is observed.