The microstructure of the heat affected zone (HAZ) in vibration welded joints of polyamide-6 were studied using polarized light microscopy (PLM), Fourier transform infrared (FTIR) microspectrometry, scanning electron microscopy (SEM), and modulated differential scanning calorimetry (MDSC). PLM images showed the existence of two distinct HAZ layers: an inner layer (HAZ-I) with small spherulites and an outer deformed layer (HAZ-II) adjacent to the bulk zone. The thickness of the HAZ-I and HAZ-II layers and the size of spherulites in HAZ-I depended on welding parameters, such as weld pressure and amplitude. The microstructure study of molecular orientation, using birefringence, and crystal phase type, using FTIR, suggests that HAZ-I originates from a molten polymer film, while HAZ-II arises in the polymer in the rubbery state. Etched HAZ surfaces also show clear distinction between the two HAZ layers. Therefore, HAZ-I is renamed as the recrystallized layer and HAZ-II as the deformed layer. Fracture tests show that the weakest part of the joint occurs in the recrystallized layer region adjacent to the deformed layer. MDSC analysis indicates that the determining factor of the weld strength is the crystallinity of the recrystallized layer, not the thickness of HAZ. POLYM. ENG. SCI., 48:240-248, 2008. (c) 2007 Society of Plastics Engineers.