Integrating MoS2 with carbon-based materials, especially graphene, is an effective strategy for preparing highly active non-noble-metal electrocatalysts in the hydrogen evolution reaction (HER). This work demonstrates a convenient hydrothermal method to fabricate molybdenum disulfide nanosheets/nitrogen- doped reduced graphene oxide (MoS2/NGO) hybrids using polyoxomolybdate as the Mo precursor. Introducing more defects and expanding interlayer spacing of MoS2 can be achieved through decreasing the pH value of the reactive system due to the existed high-nuclear polyoxometalate clusters. MoS2/NGO hybrids prepared at low pH exhibit superior HER activity to those obtained at high pH. MoS2/NGO-pH1.5 exhibits an ultralow overpotential of 81 mV at 10 mA cm(-2), a low Tafel slope of 60 mV dec(-1) and good stability in alkaline electrolyte. Such excellent electrocatalytic activity is contributed by the abundant HER catalytic active sites, the increased electrochemically-accessible area and the synergetic effects between the active MoS2 catalyst and NGO support. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.