Square-wave adsorptive stripping voltammetry (SWAdSV) was used to determine iron in ethanol and biodiesel using a bismuth-film electrode (BiFE) prepared onto the surface of a glassy carbon electrode (GCE) by electrochemical deposition to promote the reduction of Fe (III) previously complexed with 1-(2-pyridylazo)-2-naphthol (PAN) directly in the electrochemical cell. The supporting electrolyte was composed by mixture of acetate buffer (0.1 mol L−1, pH 4.5) and ethanol (40/60% v/v) into which 500 µL of a 0.1 mmol L−1 stock solution of PAN was added as complexing agent. The Fe (III)-PAN complex presented a well-defined current peak at −0.7 V. For biodiesel, a treatment with tetramethylammonium hydroxide (TMAH) was proposed as an efficient mean to minimized matrix interferences. A limit of detection of 6.0 × 10−8 mol L−1 (0.06 µmol L−1) and limit of quantification of 2.0 × 10−7 mol L−1 (0.2 µmol L−1) were obtained for Fe(III). Under the optimized conditions, there were no significant interferences from Cu(II), Al(III), Mn(II), Cr(III), Cd(II), Zn(II) and Ni(II) and Pb(II) while Ni(II) interfered significantly. The analytical curves produced linear responses with equations I (µA) = (-1.315 × 10−7 ± 5.158 × 10−8) + (-0.238 ± 0.01) [Fe (III)] (µmol L−1), R2 = 0.992 and I (µA) = (-6.836 × 10−7 ± 1.124 × 10−8) + (-0.408 ± 0.013) [Fe (III)] (µmol L−1), R2=0.998 for pure ethanol and biodiesel, respectively. The method produced satisfactory results in quantifying original quantities of Fe(III) in fuel ethanol (5.65 ± 0.71 µmol L−1) and biodiesel (1.28 ± 0.25 µmol L−1) at a 95% confidence limit (n = 3).