The effect of chloride ions (Cl-) during the immersion plating of copper onto porous silicon (PS) from a methanol (MeOH) solution has been studied. The presence of Cl- in the Cu2+ solution was found to slow down the rate of copper deposition, as confirmed by inductively coupled argon plasma emission spectroscopy and X-ray photoelectron spectroscopy measurements. The threshold concentration of Cl- at which the deposition of copper is very severely diminished was found to be 0.1 M. The inhibition effect is discussed on the basis of the rest potential values of PS and polarization curve measurements. They revealed that the rest potential of PS upon dipping in these solutions appears to direct the metal deposition. Current density-potential curves show that at Cl- concentrations higher than 0.1 M, the reduction of Cu ions proceeds in two steps, the reduction of Cu(II) to Cu(I) followed by the reduction of Cu(I) to Cu(O). This suggests that Cu(I) species in MeOH solution can be stable over a certain potential range and this stability of Cu(1) is responsible for the inhibition of metal deposition. Fourier transform infrared spectroscopy and scanning electron microscopy (SEM) were also performed to investigate the structural changes and characterizations of PS samples after the plating process.