Amorphous molybdenum selenide nanoparticles were synthesized by the solvothermal treatment of Mo(CO)(6) and Se in dimethylformamide. By varying the Mo(CO)(6) over Se molar ratio, we obtained a family of MoSe nanoparticles having comparable morphology but different chemical composition. Using a combination of experimental analyses (e.g. XRD, ICP-MS, Raman, XPS) and DFT theoretical calculation, we found that the structure of MoSe was close to that of the amorphous molybdenum sulfide analogous but not that of MoSe2 layers. The MoSe was found to consist of [Mo3Se13](2-) cluster within its structure together with some structural defects. Thanks to its less ordered structure, the MoSe can be activated by several chemical and electrochemical treatment resulting in a catalytic enhancement. A treatment with fuming HCl resulted in generation of a novel catalyst displaying 1.4 time higher catalytic current. Remarkably, a treatment with reflux NaOH solution resulted in generation of a component being soluble in water and displaying catalytic H-2-evolving activity at a moderate onset overpotential of ca. 200 my, being one of the most attractives homogeneous catalysts in water. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.