Abstract Background Patients with heart failure (HF) most likely have an extensive arrhythmogenic substrate which increases vulnerability to atrial fibrillation (AF). Conduction disorders underlying this increased AF vulnerability -hidden during sinus rhythm- may be unmasked by spontaneous atrial extrasystole (AES). Objective The aim of this study is to investigate the impact of AES on potential arrhythmogenic electrophysiological properties in patients with HF and AES. Methods Intraoperative epicardial mapping of the right and left atrium (RA, LA), Bachmann’s bundle (BB) and pulmonary vein area (PVA) was performed in 132 patients (10 with HF and 122 without HF). Differences in unipolar potential morphologies and conduction disorders during SR and spontaneous AES were determined. Results A median of 1 (1–3) AES per patient were included. In patients with HF, AES were associated with lower median potential voltages (1.94mV versus 4.02mV, P<0.001), more low voltage areas (10.98% versus 4.97%, P=0.004) and a higher number of short double potentials (28.5% versus 13.0% of short double potentials, P<0.001) compared to patients without HF. Comparing differences (AES - SR) in parameters between patients with and without HF, there was a larger increase in long double potentials (+5.32% versus +0.31%, P<0.001), fractionated potential (+4.18% versus +0.49%, P=0.001), conduction block (+3.39% versus +0.63%, P=0.002) and continuous conduction delay and block (+6.72% versus +1.74%, P=0.001) in HF patients. Conclusions In HF patients, AES cause lower unipolar potentials voltages and more conduction disorders than in patients without HF. Differences in arrhythmogenic properties between sinus rhythm and atrial extrasystoles are more pronounced in HF patients. These results suggest that HF patients have hidden arrhythmogenic substrates that can be uncovered by AES and may explain the increased AF vulnerability.