Purpose: To evaluate the effect of layer thickness of flowable composites on the marginal adaptation of Class II fillings after thermomechanical loading (TML). Materials and Methods: Sixty standardized Class II box cavities were prepared under simulation of intrapulpal pressure with gingival margins located 1 mm below the cementoenamel junction (CEJ) in dentin. The samples were evenly distributed into six groups (A to F). After adhesive (XPBond) application, box preparations were filled with a first increment of either a nanohybrid composite (A, D) Ceram•X mono, or with one of two flowable materials SDR (B, E) or x-Flow (C, F). The first increments were 1 (A,B,C) or 4 mm (D,E,F) thick. All cavities were finally filled incrementally with Ceram•X mono. Replicas were prepared before and after TML (1.2 x 106 cycles; 5/50°C; maximum load 49 N). Replicas were evaluated for marginal adaptation (tooth/composite) using scanning electron microscopy (200X). The percentage of continuous margins was compared between and within groups before and after TML using ANOVA and Scheffé's post-hoc tests. Results: For group F before TML, adaptation of cervical margins located in dentin was compromised compared to the other groups (p 0.05). After TML, at the same location, group F showed significantly worse adaptation compared with groups A to C and E (p 0.05), but no difference to group D was found. The marginal integrity of all interfaces before and after TML was significantly worse in group F compared with all other groups (p 0.05). Conclusion: Different flow application techniques for Class II cavities have an influence on the marginal adaptation (before/after TML). Applying a 4-mm first increment, both Ceram•X mono and SDR showed no differences vs groups in which the first increment was 1 mm thick. Keywords: Class II cavities, flowable composite, incremental technique, SDR, UEDMA