An in vitro dye-penetration study was performed on 2 different self-etching adhesives (Prompt L-Pop II, 3M-Espe; NRC, Prime&Bond NT Quix, Dentsply/DeTrey) in comparison to conventional conditioning of the cavity with phosphoric acid (Conditioner 36, P&B NT Quix, Dentsply/DeTrey). Standardized mod cavities (one cervical margin in cementum, one in enamel) were prepared in 80 extracted human teeth (n = 20/group). Prompt L-Pop II was employed in two different sequences: A: placing the matrix band first, then applying the adhesive, B: first applying the adhesive, then placing the matrix band. The adhesive was cured in both cases. Conditioning and application of P&B NT Quix was carried out after placement of the matrix band in both procedures. The compomer Dyract AP was used as a restorative material in all cavities. After thermocycling, the median percentages of dye-penetrated (2% methylene blue) margins were determined separately for the cementum and enamel margins of the approximal boxes and for the lateral enamel margins of the boxes. The results showed that when using Prompt L-Pop II in Class II cavities, the adhesive should be applied before placing the matrix band (p 0.0001). When the matrix band was placed first, P&B NT Quix showed superior results at a statistically significant level (p 0.0001) in comparison to Prompt L-Pop II. There was no significant difference in the use of P&B NT Quix in combination with NRC or phosphoric acid in these Class II cavities. It is strongly recommended that Prompt L-Pop be applied prior to placement of the matrix band. Prime & Bond NT Quix seems to provide clinically relevant marginal seal in Class II cavities, regardless of whether NRC or phosphoric acid is used for conditioning.