Ni is electrodeposited into polycarbonate porous membrane templates at various cathodic potentials and bulk solution pH values. The membrane pore diameters are 200 nm. Transmission electron microscope observations reveal that the electrodeposited Ni nanostructures are nanowires but occasionally nanotubes. The nanotube wall thicknesses vary from 10 to 70 nm. Nanotubes with thinner walls are deposited under more acidic and less noble conditions. The numerical model for pH values at the cathode surface in the template is developed. Calculated pH values at the cathode surface are lower in the template than those at a vertical planar cathode. The current efficiency of Ni is measured by inductively coupled plasma emission spectrometry. As the predicted by the pH calculations, the current efficiency of Ni is lower for a porous template than for a vertical planar cathode. The observed nanotube walls become thinner as the current efficiency decreases. Thus, it is deduced that accompanying H-2 evolutions promote nanotube growth in the pores. Finally, the pH values in the template are more than 1 to 2 times smaller than at the vertical planar cathode surface. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3365038] All rights reserved.