Purpose: To investigate the effects of placement technique and adhesive material on adaptation of composites before and after light curing. Materials and Methods: Cylindrical cavities (3 mm diameter, 1.7 mm depth) in extracted human molars were restored in 6 groups (n = 5) using 2 adhesives - two-step self-etching Clearfil SE Bond 2 (SE2) and all-in-one Clearfil Tri-S Bond Plus (TSP) (Kuraray Noritake Dental) - and 2 composites - Estelite Sigma Quick (ESQ) and Estelite Flow Quick (FLQ) (Tokuyama Dental) - placed with three different techniques: ESQ bulk placed, FLQ lining followed by ESQ and FLQ bulk placed. Specimens were scanned twice using swept-source optical coherence tomography (SS-OCT) before and after photopolymerization of the composite. Gap formation during polymerization or the difference in floor interface (DFI%) and final unsealed interface (USI%) were measured by image coregistration and subtraction on 6 diametrical planes across each scan. Results: Two-way ANOVA suggested that both factors (adhesive and filling technique) and their interaction were significant (p 0.001). SE2 showed significantly lower DFI% than did TSP when the composites were placed in bulk, but no difference was found when flowable lining was applied (p 0.05). Within TSP, all filling techniques were significantly different and the lining group showed the lowest values, followed by ESQ-bulk. Overall, SE2 always showed lower UFI% than did TSP, while there was no difference among different techniques within SE2. Conclusion: SS-OCT is a unique method to observe the pre-existing interfacial defects and gaps developed during polymerization, which were found to depend on both placement technique and applied adhesive. Keywords: swept-source optical coherence tomography, gap formation, image subtraction, two-step self-etching adhesive, all-in-one adhesive, lining