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2 Background- Termination of persistent atrial fibrillation (AF) can be achieved through ablation, with the majority of patients terminating to an atrial tachycardia (AT) and fewer directly to SR. We aimed to identify potential predictors for the existence of a substrate for AT upon termination to sinus rhythm (SR). Methods and Results- We assessed 95 persistent AF patients (60r10 years) who underwent catheter ablation to the endpoint of AF termination. Forty patients terminated directly to SR (SRterm) and 55 to ATs (ATterm). Compared to the ATterm group, the SRterm group were younger (56r10 vs. 63r9 y, p=0.001), had shorter durations of AF before ablation (9r26 vs. 14r20 months, p<0.001), smaller left atrium (LA) diameters (41r5 vs. 45r5 mm, p=0.015) and longer baseline AF cycle lengths (AFCL, 178r23 vs. 159r31 ms, p=0.005). However, AFCL was the sole independent predictor of direct termination to SR. The most frequent AF termination site in SRterm patients were the pulmonary veins (PV, 53%), whereas in ATterm patients this was within the LA (58%). After follow-up of 12r6 months, there was a tren