Purpose: To evaluate the effect of bulk filling Class I posterior cavities on bonding to cavity-bottom dentin. Materials and Methods: Two flowable "base" bulk-fill composites (Filtek Bulk Fill Flowable, SDR), one paste-like "full-body" bulk-fill composite (Tetric EvoCeram Bulk Fill) and one conventional paste-like composite (Filtek Z100) were bonded (G-ænial Bond) to either a flat surface (3.5 x 3.5 x 4 mm; C-factor: 0.18) or a Class I cavity (3.5 x 3.5 x 4 mm; C-factor: 5.8). After 1-week water storage, the restorations were sectioned to obtain 4 rectangular microspecimens that were subjected to microtensile bond strength (μTBS) testing. Results: No significant differences in μTBS were recorded between all composites when bonded onto a flat surface (p > 0.05). When bonded into a Class I cavity, the μTBS of all composites except SDR significantly decreased (p 0.001). Conclusion: Both the configuration factor and the type of bulk-fill composite were found to have a great impact on bonding to cavity-bottom dentin. Keywords: adhesion, μTBS, shrinkage, configuration