The aim of this in vitro study was to compare the approximal marginal quality of composite fillings using a dentin bonding system to the marginal quality of hybrid ionomer/composite sandwich restorations. Forty-eight standardized class 2 cavity preparations were prepared in caries-free, human third molars. Twelve preparations at a time were filled either with a composite using the matching dentin bonding system, SZ (Scotchbond MP/Z100) or PP (PROBOND/Prisma TPH) or with hybrid ionomer/composite sandwich fillings VZ (Vitremer/Z100) or DP (Dyract/Prisma TPH). Margins were evaluated before and after thermomechanical loading (TCML) (5000 cycles [+5 degrees C/+55 degrees C], 72.5 N [1,7 Hz]) by quantitative scanning electron microscope analysis using an image analyzing system. Furthermore, microleakage was assessed by dye penetration before and after TCML. Statistical analysis was performed using the Mann-Whitney test at the 0.05 level of significance. SEM analysis after TCML showed significantly fewer marginal gaps at the material/dentin interface with VZ (2.2%), DP (7.3%), and PP (6.0%) compared to SZ (29.6%). After TCML, SZ showed significantly more marginal gaps at the material/dentin interface, whereas VZ, DP, and PP were not susceptible to TCML. PP showed the highest percentage in marginal swelling before (18.2%) and after TCML (15.9%), while VZ showed no marginal swelling at all. VZ showed significantly fewer marginal gaps at the composite/hybrid ionomer interface than DP. After TCML there was no significant difference in marginal gaps between the hybrid ionomer/enamel and the hybrid ionomer/dentin interface for both Vitremer and Dyract. Using the sandwich technique the Z100/enamel interface had significantly more marginal gaps than all other composite/enamel interfaces after TCML. There was no significant difference in microleakage between the test groups at the material/dentin interface. SEM analysis and dye penetration showed that hybrid ionomer/composite sandwich restorations have good marginal qualities and may be an alternative to composite restorations using a dentin bonding system.