Analysis of a cobalt bis(dithiolate) complex reported to mediate hydrogen evolution under electrocatalytic conditions in acetonitrile revealed that the cobalt complex transforms into an electrode-adsorbed film upon addition of acid prior to application of a potential. Subsequent application of a reducing potential to the film results in desorption of the film and regeneration of the molecular cobalt complex in solution, suggesting that the adsorbed species is an intermediate in catalytic H-2 evolution. The electroanalytical techniques used to examine the pathway by which H-2 is generated, as well as the methods used to probe the electrode -adsorbed species, are discussed. Tentative mechanisms for catalytic H-2 evolution via an electrode-adsorbed intermediate are proposed.