Extrusion can yield expanded products or non-expanded pellets, which are later expanded in a separate thermal device to form third-generation (3 G) snacks. In both processes, ingredient effects on structure formation and functional ingredient retention are challenging. This study analyzed the effect of adding 0-15 % grape pomace flour (GPF), a by-product of the wine industry, to rice flour pellets extruded at maximum temperatures of 110 and 150 degrees C using twin-screw extrusion and then microwave-expanded (30 s at 480 W), to understand the relationship between microstructural parameters of 3 G snacks and their physical and textural properties, total polyphenol content (TPC), and antioxidant capacity. Results showed that adding GPF increased bulk density (from 0.12 to 0.39 g/cm3), hardness (from 29.5 to 37.59 N), and crispness, while decreasing the expansion index (from 1.83 to 1.65). Micro-CT analysis revealed that extrusion temperature did not affect microstructural parameters, but high GPF levels (15 %) reduced porosity and mean pore size, and increased wall thickness. Extrusion reduced the initial TPC in pellets by up to 52 %, but microwave expansion increased it by up to 37 %, likely due to polyphenol release from the dense glassy matrix of the pellets. These findings highlight the potential for creating nutritionally enhanced 3 G snacks using agricultural by-products like GPF and underscore the importance of a microstructural approach in the analysis.