The continuous solid solution NiFe2-xMnxO4 (0 <= x <= 2) prepared by sol-gel is successfully tested for the hydrogen production under visible light. All spinels crystallize in a cubic structure with a particle size ranging from 30 to 63 nm. They exhibit p-type conductivity with an average thermopower of similar to 135 mu V K-1, a hole mobility of similar to 9 x 10(-5) cm(2) V-1 s(-1) and an activation energies in the range (0.12-0.33 eV). The optical transition of the most active photocatalyst NiFe1.6Mn0.4O4 (1.53 eV) is well matched to the solar spectrum. The chemical and physical properties are correlated to assess the feasibility of the spinels for the hydrogen liberation. The conduction band (-1.54 V-SCE) is located below the H2O/H-2 potential (similar to-0.8 V-SCE), allowing H-2-evolution under visible light. The spinel dose, pH and S2O32- concentration are optimized. Under the ideal conditions, the volume of H-2 liberation reaches 124 mu mol at saturation after 20 min of irradiation. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.