The purpose of this study was to evaluate the effect of thermocycling on a colored glass ionomer intracoronal barrier used for the prevention of microleakage. Thirty single canal premolars were decoronated, standardized in length, instrumented, obturated, and randomly assigned to three groups. Group 1 received a 1 mm intracoronal barrier of Triage glass ionomer, group 2 received a 2 mm Triage barrier, and group 3 received no barrier. After incubation for sealer set, teeth were thermocycled. Microleakage was measured using the fluid transport model. Groups 1, 2, and 3 demonstrated 1.68 mm, 0.60 mm, and 23.24 mm of movement, respectively. Using ANOVA and Student-Neumann-Keuls, group 3 leaked significantly more (p < 0.05) than groups 1 and 2, with no difference between groups 1 and 2. A 1 or 2 mm intracoronal barrier of Triage significantly reduced coronal microleakage in thermocycled endodontically treated teeth.