Nano-structured magnetites (C-Fe3O4, G-Fe3O4 and Au/G-Fe3O4) were synthesized in the absence and presence of aqueous leaf extract of Averrhoa carambola, a natural, nontoxic surfactant and reductant. The magnetites were characterized by XRD, BET surface area, UV vis DRS, TEM, FESEM, FT-IR, XPS and PL for structural, morphological and optical properties. The photocatalytic activities of magnetites were evaluated for the hydrogen generation by decomposition of water and reduction of Cr (VI) under visible light irradiation. Nano-structured magnetite (G-Fe3O4 and Au/G-Fe3O4) prepared in the presence of bio-surfactants showed higher photocatalytic activity compared to magnetite (C-Fe3O4) synthesized in absence of it. Among all the prepared catalysts, the in situ loaded Au nanoparticles over G-Fe3O4 (Au/G-Fe3O4) showed higher catalytic activity due to its surface plasmon resonance effect. The novelty of this present study is the synthesis of highly efficient photoactive magnetites of high surface area using a natural surfactant at room temperature in a one step method without using any external stabilizing agent and reducing agent. (C) 2016 Elsevier B.V. All rights reserved.