Amorphous molybdenum sulfide films, prepared by electrodeposition, are a class of highly active catalysts for hydrogen evolution. The growth mechanism of the films and the true active species were unclear. Herein, we report a study of the growth and activation of these films using Electrochemical Quartz Crystal Microbalance (EQCM) and X-ray photoelectron spectroscopy (XPS). Three processes, including oxidative deposition, reductive corrosion, and reductive deposition, are occurring during the growth of a molybdenum sulfide film. Deposition method, precursor concentration, and potential window are among the factors influencing the film growth. Regardless of deposition methods, all films exhibit similar catalytic activity on a per mass base. Potentiostatic oxidation (anodic electrolysis) is the method for fastest film growth; it produces a MoS3 film precatalyst which can be electrochemically activated. The activity of the MoS3 precatalyst scales with catalyst loading; at a loading of 0.2 mg/cm2, the current density is 20 mA/cm2 at an overpotential of 170 mV. Films differently deposited have different initial compositions, but the active catalysts in all films are the same MoS2+x species, whose XPS characteristics are distinct from those of crystalline MoS2. The activation process of a MoS3 film precatalyst involves a reductive removal of slightly less than one equivalent of sulfide to form MoS2+x.