We report on the mechanism of porous silicon (PS) formation from two different fluorohydrogenate ionic liquids. At the first stage of the study, an unexpected formation of PS by the anodic polarization of n-Si in 1-ethyl-3-methylimidazolium (FH)(2.3)F occurred without illumination. X-ray photoelectron spectroscopy studies show that the PS formation mechanism is based on the Si-F bond formation at the Si/electrolyte interface. At the second stage of the study, the formation of PS from n-Si in Cs(FH)(2.3)F under anodic polarization was studied without illumination as well. X-ray diffraction studies of the precipitated white film on the silicon wafer during anodic polarization revealed the formation of crystalline Cs2SiF6. A suggested mechanism of PS formation in the ionic liquids is proposed.