NiS was loaded on ZnS1-x-0.5yOx(OH)(y)-ZnO photocatalyst by two methods (depositing NiS on the photocatalyst via impregnation with a NiS sol and via a precipitation reaction). The activity of the NiS-modified photocatalyst for hydrogen evolution from pure water and seawater was investigated using mixed electron donors (sulfide and sulfite). The activity (in pure water-electron donor solution) is dependant of the modification method and loading content of NiS. When Na2S center dot 9H(2)O and Na2SO3 are added into seawater, a mixture precipitate (Mg(OH)(2) and CaSO3) produces. The precipitate is detrimental to the photocatalytic hydrogen evolution over the unmodified ZnS1-x-0.5yOx(OH)(y)-ZnO. However, for the NiS-modified photocatalyst, the detrimental effect decreases notably. Interestingly, the activity of the modified photocatalyst in seawater-electron donor solution (with the precipitate) is higher than that in the pure water-electron-donor solution (without the precipitate). The possible mechanism was discussed. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.