Purpose: The objective of this study was to investigate the effect of operator variability on the marginal performance of resin composite restorations bonded with an acetone-base one-bottle adhesive in standardized dentin preparations. Materials and Methods: Five general practitioners were recruited for this trial. In the first group, each dentist received six extracted human teeth with dentin preparations (0 3.5 mm, 1.5 mm deep), cut in flat-ground proximal dentin, and original packages of Gluma One Bond adhesive and Charisma resin composite (Heraeus-Kulzer, Werheim, Germany) including instructions for use. For the second group, the operators were orally instructed, emphasizing the importance of the moist technique, before they received another six teeth for restoration with the same materials. Finally, a third group of six preparations was restored in order to evaluate a possible training effect. The restored teeth were stored in a hygrophor prior to removal of excess material and microscopic determination of the maximum marginal lap width (MGW). Statistical analyses were performed with the Kruskal-Wallis and Wilcoxon tests at p < 0.05. Results: Apart from two samples, preparation margins in the first test group showed gaps between 1 and 12 µm. In the second and third groups, 15 and 16 restorations were gap free, respectively. Significant interoperator variation was found in the first group only. Marginal performance of group 1 restorations was significantly inferior to groups 2 and 3, which were not different (p = 0.79). Conclusion: Application of the one-bottle adhesive is technique-sensitive and requires meticulous attention to the instructions.