The reaction mechanism of Si electrodeposition in the ionic liquid trimethyl-n-hexylammonium bis(trifluoromethylsulfonyl)imide (TMHATFSI) with SiCl 4 was investigated using an electrochemical quartz crystal microbalance (EQCM) and X-ray photoelectron spectroscopy (XPS). The deposited films were further characterized by Raman spectroscopy. The EQCM method measured the frequency change in situ during electrodeposition, whereas XPS provided the Si mass concentration in the electrodeposited films. By combining these results, the mass change due to SiCl 4 reduction was estimated. Then, the current efficiency for Si electrodeposition at various potentials was evaluated. The calculated current efficiency was almost 100% when it was assumed that SiCl 4 reduction progressed by four-electron electroreduction. The XPS and Raman results showed that the deposited films were composed primarily of amorphous Si with Si-Si bonds. Crystalline Si thin films were obtained after annealing in an Ar gas stream.