Method and Materials: Thirty-two freshly extracted caries-free human third molars were used. A wide mesio-occlusal-distal (MOD) cavity was prepared in each third molar, with the occlusal dimension being approximately twothirds of the intercuspal dimension, and the cervical limit of one proximal box was stopped 1 mm below and the other 1 mm above the cementoenamel junction. The cavities were filled with a methacrylate-based nanohybrid composite (Grandio, VoCo) and a silorane-based microhybrid composite (Filtek Silorane, 3M ESPE). All samples were subjected to 1000 thermal cycles of 5°C/55°C in water with a 30-second dwell time and, after the procedure, the teeth were immersed in a 0.5% basic fuchsin dye at 23°C for 24 hours. Sectioned samples were examined under a stereomicroscope (Leica MZ12, Leica Microsystems), and microleakage scores were statistically analyzed using the Kruskal-Wallis and the Mann-Whitney U tests. Clinical RelevanceThis in vitro study showed that the Silorane-based microhybrid resin composite system had no microleakage for wide Class II MOD restorations with oblique and vertical layering techniques.