Porous silicon (P-Si) is a suitable material for applications in gas sensors, biomedical sensors and optical sensors. In this work, the effects of humidity and acetone on the optical and electrical properties of porous silicon nanostructures, produced by two different methods, are studied. The first method is stain etching of silicon wafers by evaporation of hydrogen fluoride (HF) based etchant. The second method is the electrochemical anodization of silicon wafers at low current densities in HF-based solutions under white light. Electrical conduction mechanism of P-Si samples is examined through AC and DC measurements. It is observed that the I-V response of the samples shows a rectifying behaviour that is dependent on the ambient relative humidity (rh). AC measurements show that such samples are quite capacitive and this behaviour shows a very little dependence on the relative humidity. Photoluminescence (PL) spectroscopy measurements are performed with pumping the samples with laser wavelengths of 313.8 nm and 365.6 nm. The PL peak position is observed between 600-650 nm, depending on the sample preparation conditions. The intensity of the luminescence decreases by time and the peak wavelength makes a red-shift due to oxidation. Photoluminescence spectroscopy measurements are also repeated after exposing the samples to water and acetone. It is observed that water decreases the luminescence intensity very little and causes a slight red-shift while acetone decreases the intensity more and causes no important shift in the peak wavelength. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.