Immersion plating of nickel (Ni) on a porous silicon (PS) layer has been investigated in concentrated ammonium fluoride (NH4F) and dilute hydrofluoric acid (HF) solutions containing Ni2+. When the PS sample was exposed to the bath containing 5 M NH4F and 50 mM Ni2+ at pH 8, metallic Ni was clearly observed at the open-circuit potential without using a reducing agent or any activation treatment at room temperature. However, no deposition was detected when the sample was immersed in a 0.5 wt % HF solution containing the same concentration of Ni2+ at pH 2. The different deposition behavior is discussed on the basis of mixed potential theory, changes in the stability of Si-H bonds of PS as indicated in the Fourier transform infrared spectra, and the different state of Ni complex formation as obtained from the UV-vis spectra. The position of the Ni redox levels in both solutions with respect to the bandedges of Si was also determined and the results revealed a nearly similar energetic situation. Results from current-potential curves showed that a slow Ni deposition from the alkaline solution occurred in the dark and different possible reaction mechanisms were proposed. They also revealed the enhancement of the deposition under illumination, indicating that the deposition mechanism is electron transfer from the conduction band. X-ray photoelectron spectroscopy detected no silicon oxides on the PS surface after being immersed in the alkaline solution, giving merit that a binary PS/Ni nanostructure without Si oxides can be successfully achieved. (C) 2003 The Electrochemical Society.