We have undertaken a systematic study of porous silicon with the use of photoconductivity and photoluminescence. During this, we have observed at least three kinds of time-varying photoconductivity, of which two reduce the conductivity and one increases the conductivity during illumination. In addition, we have observed persistent photoconductivity. The time developments of the photoconductivity as well as the persistent photoconductivity depend in intricate ways on parameters such as the wavelength and intensity of the illuminating light, the potential drop across the sample, the surface treatment, and the dark current value. The time scale of these time-varying effects ranges from a few minutes to several hours. The results are discussed in terms of the photoelectric properties of the supporting silicon wafer, diffusion of hydrogen, and photoinduced desorption of hydrogen. ©1997 American Institute of Physics.