The success of external cardioversion (ECV) of atrial fibrillation depends on generating sufficient transmyocardial current for defibrillation with minimal myocardial injury. Thoracic electrical impedance plays an important role in the relation between the delivered energy and transmyocardial current. This study assessed the determinants of thoracic electrical impedance in ECV of atrial fibrillation. ECV of atrial fibrillation was performed in 80 consecutive patients (mean age 73 ± 9 years; men 69%; body mass index 26.0 ± 3.6 kg/m²) within 12 months, using biphasic shocks (Multipulse Biowave) delivered through adhesive pads in an anteroposterior position. Thoracic electrical impedance was measured using the first shock. The mean thoracic electrical impedance was 57.7 ± 12.3 Ω (energy 71 ± 43 J, current intensity 33 ± 12 A). Sinus rhythm was immediately restored in 75 patients (94%). Thoracic electrical impedance was greater (60.9 ± 11.8 vs 51.7 ± 11.0 Ω, p = 0.001) in patients requiring >1 shock (65%). At multivariate linear regression analysis (R = 0.761, p <0.001), female gender (+9.7 ± 2.0 Ω, p <0.001), body mass index (+1.5± 0.3 for a 1 kg/m² increase, p <0.001), hemoglobin concentration (+1.9 ± 0.6 for a 1 g/dl increase, p = 0.004), and the presence of chronic heart failure (−5.3 ± 2.0 Ω, p = 0.009) were independent predictors of thoracic electrical impedance. In conclusion, to increase ECV efficacy and minimize complications, the delivered energy should be adjusted in accordance with the clinical variables that independently affect thoracic electrical impedance and, hence, transmyocardial current.