Replacing Pt by earth abundant catalysts is one of the most important tasks toward potential large-scale HER applications. Among many potential candidates, low cost and earth abundant transition metal dichalcogenides such as MoS2 and WS2 have been promising as good H-2 evolution electrocatalysts when they are engineered into the structures with active sites. In this work, we have performed systematic studies on the catalytic reactivity of both MoS2 and WS2 materials produced by one-step and scalable thermolysis from (NH4)(2)WS4 and (NH4)(2)MoS4 precursors respectively. Structural analysis shows that these materials prepared at a higher thermolysis temperature exhibit higher crystallinity. The H-2 evolution electrocatalysts efficiency for the MoS2 prepared at a lower temperature is higher than those at higher temperatures, where amorphous MoS2 or S-2(2-) species instead of crystalline MoS2 is the main active site. By contrast, crystalline WS2 prepared at high temperature is identified to be the key reaction site. Both catalysts display excellent efficiency and durability as an electrocatalyst operating in acidic electrolytes. This work provides fundamental insights for further design and preparation of emergent metal dichalcogenide catalysts, beneficial for the development in clean energy. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.