Transcutaneous pericardial instrumentation in a patient with a left-sided posteroseptal accessory pathway associated with a large coronary sinus diverticulum was performed after three previous unsuccessful ablations. The earliest ventricular activation site was recorded epicardially using a catheterin the pericardial space, but energy delivery at this site was not possible due to high impedances. Intravascular linear lesions transecting the neck of the large coronary sinus diverticulum using the pericardial catheter as a target allowed successful radiofrequency catheter ablation of the accessory pathway. This case outlines potential problems with ablations in the pericardial space and provides an alternative solution that can spare patients from open surgical procedures.