In the past 45 years, the search for semiconductors as active photocatalysts for overall water splitting has focused on insoluble materials and their hybrids. An important question is whether soluble semiconductors have the capacity for photocatalysis or similar applications. The dissolved semiconductors will lose the energy band structures for light absorption; however, the undissolved part in saturated solution can still generate electrons and holes under illumination. Unfortunately, this possibility has never been realized. Here we clearly demonstrate the use of a water-soluble sodium molybdate salt as an effective photocatalyst. The material can photocatalyze simultaneously the oxidation and reduction of water under band-gap irradiation. We anticipate that, as a large and traditional class of chemical compounds, the soluble semiconductors may have great potential to be applied in numerous important applications such as catalysis, photovoltaics, light emitting diodes and artificial photosynthesis. (C) 2017 Elsevier B.V. All rights reserved.