The stepwise approach to radiofrequency (RF) ablation of atrial fibrillation (AF) can include ablation of the coronary sinus (CS) by RF delivery at the left atrium (LA) and/or within the CS. In both cases, the energy is applied between the tip electrode of a percutaneous catheter and a dispersive electrode on the body surface. We explored the feasibility of using the electrode rings of a diagnostic catheter placed in the CS as dispersive electrode(s) for RF delivery within the LA and compared this technique to an established CS ablation method.Excised pig hearts were superfused with a pulsatile saline flow. Bipolar ablation was performed between a saline-irrigated (20 mL/minute) 4-mm tip electrode placed in the LA adjacent to the CS and 7 electrode rings of a 6F, septapolar, 4-mm nonirrigated electrode placed within the CS adjacent to the LA endocardial electrode. Unipolar ablation was performed between the endocardial electrode and dispersive electrode. A continuous transmural lesion was produced in 6/8 (75%) attempts with bipolar ablation, but in 0/6 (0%) attempts with unipolar ablation. However, the incidence of steam pop tended to be increased with bipolar ablation.Bipolar ablation of the CS appears to be highly effective for creating a transmural LA-CS lesion.