Electrodeposition of Ni in a Watt's bath at different applied pressures and in the presence of supercritical CO2 (sc-CO2), either with or without surfactant addition, was investigated. The current efficiency was evaluated under constantly applied current density conditions. The crystal structure of the resulting Ni film was characterized by performing X-ray diffraction. The composition of the deposit was analyzed using X-ray photoelectron spectroscopy. Transmission electron microscopy was employed for microstructure analysis. Microhardness of the deposited film was measured to distinguish the role of bath composition. The experimental results showed that carbon-containing nanocrystalline Ni films could be obtained in the bath with the presence of sc-CO2. A significant increase in microhardness was found for the film electrodeposited in sc-CO2 fluid, as compared with that formed in plain aqueous electrolyte. The fine grain size and solid solution strengthening caused by carbon were responsible for the increased hardness. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3205876] All rights reserved.