Objective: The purpose of this study was to determine the effectiveness of mineral trioxide aggregate and glass ionomer when placed coronally as double-sealing materials over gutta percha. Method and materials: Seventy extracted anterior teeth were cleaned, shaped, and obturated with gutta percha and Sealapex. After removing 4 mm of coronal gutta percha, the teeth were randomly divided into 3 groups. In two experimental groups of 30 teeth each, 4 mm of either mineral trioxide aggregate or glass ionomer was placed in the chamber over gutta percha. A positive control group of 5 teeth received no barrier. A negative control group of 5 teeth was covered completely with sticky wax. All teeth, except the negative controls, were covered with 2 layers of sticky wax except for the access openings. Teeth were immersed in Pelikan ink for 48 hours, and then were decalcified, dehydrated, and cleared. Leakage into the canals was measured in millimeters and statistically analyzed between the two experimental groups using the Mann-Whitney test. Results: Results showed that the glass ionomer group leaked significantly more than the mineral trioxide aggregate group (P .001). Conclusion: It was concluded from this study that mineral trioxide aggregate may be preferred over glass ionomer as a seal intracoronally following root canal treatment to prevent coronal microleakage. Schlagwörter: coronal seal, coronal leakage, dye leakage, glass ionomer, mineral trioxide aggregate, root canal treatment