Processes and compositions for electroplating a cobalt deposit onto a semiconductor base structure comprising sub-micron-sized electrical interconnect features. In the process, a metalizing substrate within the interconnect features is contacted with an electrodeposition composition comprising a source of cobalt ions, an accelerator comprising an organic sulfur compound, an acetylenic suppressor, a buffering agent and water. Electrical current is supplied to the electrolytic composition to deposit cobalt onto the base structure and fill the submicron-sized features with cobalt. The process is effective for superfilling the interconnect features.