Novel heterogeneous structure of BiFeO3-CdWO4 with different molar ratios was applied for the photocatalytic hydrogen evolution in a self-designed externally UV/visible irradiated photoreactor in the presence of potassium iodide. The photocatalysts were synthesized by simple hydrothermal method and characterized by XRD, FE-SEM-mapping, TEM, UV-Vis DRS, PL, EIS, transient photocurrent and Mott-schottky techniques to identify the structural, optical and photoelectrochemical properties. The slope of Mott-schottky plots confirmed the p-type and n-type conductivity of the synthesized BiFeO3 and CdWO4, respectively. The p-n heterojunctions exhibited more efficiently light absorption, charge separation and electron mobility relative to the pure photocatalysts. We observed that coupling 40 mol% BiFeO3 with CdWO4 provided the best photocatalytic performance of hydrogen evolution, 268.90 mu mol h(-l).g(cat)(-1) from distilled water and 379.43 mu mol h(-1).g(cat)(-1) from 0.05 M KI aqueous solution. Iodine species increased H-2 evolution efficiency because of taking part in the charge transfer processes, either by scavenging excited holes or by direct reduction of H+ to H-center dot under UV irradiation. Fermi level equilibrium in the p-n heterojunction suggests the best interparticle charge transfer mechanism explaining how photoinduced electrons with superior energy states and desirable lifetime can be supplied to reduce H+ to H-center dot. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.