Purpose: To evaluate the marginal microleakage and the infiltration ability of pit-and-fissure sealants by applying the conventional sealing technique in comparison to using an additional bonding agent. Materials and Methods: Extracted non-carious permanent molars (n = 60) were first stored in sterile saline solution and then assigned to one of two groups: group C (control) was sealed (Helioseal F) by using the conventional technique, while in group BA (bonding agent), a bonding agent (OptiBond FL) was additionally applied prior to sealing. The teeth were thermocycled (1000 cycles, 5°C to 55°C, dwell time 30 s), then varnished and immersed in 5% methylene blue solution for 24 h. After embedding and sectioning each tooth into 6-12 slices, the presence of microleakage, unfilled areas, and air bubbles trapped in the sealant were assessed with a stereomicroscope. Results: A higher proportion of microleakage was found under sealants applied without the additional use of the bonding agent. A statistically significant difference in microleakage was noted between the groups (p = 0.045). Regarding the presence of unfilled areas, a statistically significant difference between the groups was observed (p < 0.001), especially since no unfilled areas were found at all in the samples of the group using the bonding agent. Regarding the amount of air bubbles trapped in the sealant, no statistically significant difference was observed between the two groups (p = 0.829). Conclusion: Under these in vitro conditions, sealant procedures using an additional bonding agent applied beforehand significantly improved fissure infiltration and microleakage prevention significantly. Keywords: sealant, bonding agent, adhesive, primer, microleakage, unfilled area, pit and fissures