We present a thorough assessment of the compositional dependent hydrogen evolution reaction (HER) activity and stability for Co-based mixed chalcogen, CoSxSe2-x, transition metal dichalcogenides (TMDs). In direct contrast to other reports on mixed chalcogen TMDs that have been limited to Mo and W, we observe a decrease in HER activity for any departure from the pure, single chalcogen composition, CoS2 and CoSe2. The single chalcogen TMDs are found to have nearly identical HER activity. Bulk resistivity of the pure and mixed chalcogen TMDs as well as the charge transfer resistance for the HER are found to be most optimal for CoSe2. However, CoSe2 is predicted to have an endothermic hydrogen adsorption free energy in contrast to the slightly exothermic hydrogen adsorption free energy for CoS2. This highlights the convolution of hydrogen adsorption free energy and material conductivity in determining the HER activity for even metallic conducting TMDs. Sulfur-rich Co-based TMDs with a pyrite type crystal structure are found to quickly deactivate through loss of Co and formation of passivating reduced sulfur species at the surface while Se-rich compositions are observed to be more stable. Therefore, with an HER activity matching that of CoS2, but with a dramatic improvement in stability, CoSe2 breaks away from the traditional inverse activity - stability relationship and represents a promising non-PGM HER electrocatalyst for acidic PEM electrolyzers. (C) 2018 Elsevier Inc. All rights reserved.