Dense CO2 was applied in electrodeposition of Co films to examine its effects on the morphology, crystal structure, average grain size, and surface roughness. The dense CO2 is referring to CO2 in liquid state (15 MPa and 25 degrees C), subcritical state (15 MPa and 30 degrees C), and supercritical state (15 MPa and 40 degrees C) here. The electrodeposition with dense CO2 was conducted in a high-pressure cell, and the electrolyte was a mixture composed of 20 vol.% of the dense CO2 and 80 vol.% of the Co electrolyte. Influences of the temperature and applied current density were also studied. The dense CO2, especially supercritical CO2, was found to be effective in eliminating defects formed in the Co films. Morphology of the Co films changed from ridge-like to hemi-spherical-like and particle-like structures with a decrease in the temperature and an increase in the applied current density. The average grain size was found to decrease with an increase in the current density. The Co films electrodeposited at atmospheric pressure were mainly composed of h.c.p. phase and the crystal structure changed to f.c.c.-rich phase when the dense CO2 was used. (C) 2015 The Electrochemical Society. All rights reserved.