©Operative Dentistry, 2010, 35-2, 187-193 M Khoroushi • M Tavasoli Clinical RelevanceTCA gel etches enamel surfaces and produces surface morphological characteristics and bond strength similar to that produced by phosphoric acid. When TCA is used as a hemostatic agent on marginal gingiva, its inadvertent contact with enamel improves the immediate bond strength of resin composite to enamel. However, when using phosphoric acid and 50% TCA, an erosive enamel surface is produced. SUMMARYIntroduction: Trichloracetic acid (TCA) is a soft tissue chemical cauterizing agent that is used on gingival margins prior to restoring cervical cavities with resin materials. This study evaluated the effect of TCA gel as an etchant, its use before etchant on the shear bond strength between resin composite and enamel and also its effect on enamel surface morphological characteristics. Materials and Methods: Seventy-five sound, extracted human anterior maxillary teeth were selected for the purpose of this in vitro study. The teeth were equally divided into five groups prior to enamel surface preparation with silicone carbide papers. In Group 1, the enamel surfaces were etched with 35% phosphoric acid for 30 seconds. In Groups 2 and 3, a 35% TCA gel and 50% TCA gel, respectively, were used on the enamel surfaces for 30 seconds. The enamel surfaces were then rinsed with water for 10 seconds. In Groups 4 and 5, the specimens were prepared in the same manner as Groups 2 and 3 and the enamel surfaces were then In all the experimental groups, after rinsing and drying the samples, Single Bond adhesive (3M ESPE) was used to bond Z250 composite cylinders onto the enamel surfaces.After 500 rounds of thermocycling, the composite cylinders were loaded to failure in shear in a DARTEC test machine and the data were analyzed using the ANOVA and Scheffé's tests (α α=0.05). Two specimens from each group were prepared for surface morphology evaluation under SEM. Results