In this study, the growth mechanism for electrodeposition of hydrogenated amorphous carbon (a-C:H) films was discussed from two novel perspectives. First an electrical double layer is considered to form in the vicinity of the electrodes. The double layer is believed to govern the variation tendency of the external current density. Then by introducing a concept called thermodynamic coupling we have explained the growth mechanism of a-C:H films from liquid phase electrodeposition and pointed out that hydrogen atoms play a crucial role in lowering applied voltage. Related experiments are conducted to corroborate our opinion. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.034212jes] All rights reserved.