Electrical conductivity of 28 and 220-nm thick silicon membranes was measured by the van der Pauw method in dry air (relative humidity <5%) at room temperature (around 20 degrees C). Immediately after hydrofluoric acid immersion the conductivity increases several orders of magnitude because of surface-induced band bending; it then drops and reaches the level of samples with a native oxide surface in several weeks due to the surface's re-oxidation. The oxidation rate is found to increase with the de-ionized water rinsing time, which is confirmed by X-ray photo electron spectroscopy measurements. (C) 2011 Elsevier B.V. All rights reserved.