The origin of spontaneous oscillations as observed in the Cu(II)-lactate system is not known in any detail. In the present investigation, local pH changes close to the cathode, where spontaneous potential oscillations were induced from the Cu(II)-lactate system, have been measured in situ using microband electrodes coated with a pH-sensitive conducting polymer, polyaniline (PANI). The shape of the pH response has a similar appearance as the self-oscillations themselves and gives additional information about the oscillating system. The absence of a buffering system in the pH region between 10.5 and 12 has a crucial role in inducing the positive potential shift seen during the oscillations. For the negative potential shift, three different models are discussed; (1) the depletion of hydroxide ions in the diffusion layer due to cuprous oxide formation, (2) the depletion of Cu(II) ions in the diffusion layer during cuprous oxide formation, and (3) the depletion of hydroxide- and Cu(II) ions in the diffusion layer as a result of a local precipitation of Cu(OH)(2) (s) in the diffusion layer. (C) 2003 Elsevier Science B.V. All rights reserved.