A systematic study of the electrochemistry of hydrogenated amorphous silicon (a-Si:H) is presented in order to explain pore formation in this material. The similarity of the cyclovoltammograms of amorphous and crystalline silicon demonstrates that the basic electrochemical reactions are similar in both materials. A striking difference between the electrochemistry of these two materials is the existence of an instability which limits the maximum thickness of the porous amorphous material that can be obtained. We explain this instability by the large value of the resistivity of amorphous silicon in comparison with that of the electrolyte, and model this phenomenon by a linear stability analysis. We detected the same phenomenon during anodization of highly resistive p-type crystalline silicon. The photoluminescence intensity of porous a-Si:H is quite comparable with that of crystalline silicon of the same thickness.