Compositions and methods of using such compositions for electroplating cobalt onto semiconductor base structures comprising submicron-sized electrical interconnect features are provided herein. The interconnect features are metallized by contacting the semiconductor base structure with an electrolytic composition comprising a source of cobalt ions, a suppressor, a buffer, and one or more of a depolarizing compound and a uniformity enhancer. Electrical current is supplied to the electrolytic composition to deposit cobalt onto the base structure and fill the submicron-sized features with cobalt. The method presented herein is useful for superfilling interconnect features.