Nanostructured zinc ferrites with different excess iron contents (ZnFe2+zO4, where z = 0.00, 0.05, 0.10 and 0.15) have been synthesized using the sol–gel auto-combustion method. The effect of excess iron on the structural, optical and visible light photocatalytic activity of zinc ferrite samples has been investigated. X-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), x-ray magnetic circular dichroism (XMCD), Brunauer–Emmett–Teller theory, scanning electron microscopy (SEM), diffuse reflectance spectroscopy (UV–Vis) and photoluminescence spectroscopy were used to characterize the synthesized non-stoichiometric ZnFe2O4 powders. The XRD patterns demonstrated that the samples consist of single phase spinel structure with crystallite sizes of 25 nm. SEM analysis indicated that the nanosized particles grow together in porous clusters with a size of several microns. The XPS and XMCD analyses revealed that the excess iron ion substitutes Zn2+ in tetrahedral sites and in octahedral sites Fe2+ can be found in addition to Fe3+, which could be created to restore the overall charge balance in the crystal lattice. Stoichiometric zinc ferrite (ZnFe2+zO4, z = 0.00) exhibited higher photocatalytic activity (40%) than the excess iron sample (ZnFe2+zO4, z = 0.15, i.e. 4%) under visible light irradiation for 3 h. This was explained by the formation of Fe2+ in the octahedral sites of excess iron ZnFe2O4; these species act as recombination centres.