Purpose: The aim of this study was to evaluate the effect of dental conditioners in terms of dentin micromorphology and the thickness of the hybrid layer obtained by using the self-etching adhesive systems Adhe SE (AD), G Bond (GB), Adper Prompt L Pop (LP) and Tyrian (TY) and the etch-and-rinse Single Bond (SB). Materials and Methods: Dentin sticks from 50 third human molars were obtained by first cutting dentin disks out of crowns; the disks were subsequently sectioned into sticks. The sticks were assigned to experimental groups as follows: G1: phosphoric acid 37%, Single Bond (SB); G2: Adhe SE (AD); G3: G Bond (GB); G4: Adper Prompt L-Pop (LP); G5: Tyrian SPE (TY). Half the samples served to investigate the dentin micromorphology. To this end, the proprietary conditioner of the respective adhesive system was applied to these samples, and conditioner effects were examined with SEM. The other half of the samples was hybridized and restored with composite resin to investigate the thickness of the hybrid layer, which was also evaluated with SEM. Statistical analysis was performed with ANOVA, Tukey's, and the Kruskal-Wallis tests. Results: SEM observation showed significant differences of the hybrid layer thickness. LP presented the highest average thickness, 3.45 µm, while SB presented an average of 2.44 µm and AD an average of 1.43 µm. The averages of TY and SB did not show any statistically significant differences. The adhesive system GB presented the lowest average in hybrid layer, 0.37 µm. The micromorphological evaluation showed that the conditioners commonly removed the smear layer. Conclusion: All the micromorphological alterations and the hybrid layer thickness show that the adhesive systems studied performed distinct actions. Although there is no apparent correlation of the pH of the conditioners with the thickness of the hybrid layer, it is clear that more acidic conditioners promote more severe transformations in the dentin micromorphology. Keywords: adhesion, dentin bonding agent, dentin, surface modification