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Abstract The interest in developing fast and reliable chemical and biochemical sensors in an inexpensive way is something that has attracted a lot of efforts in the last decades. One of the potential material candidates is electrochemically treated silicon to form porous silicon (PSi). This treatment generates silicon nanostructures within the porous matrix. The large area to volume ratio is especially interesting for sensing applications. The main drawback of this material is the fast oxidation that impedes the correct device detecting capability. This work is devoted to the study of the oxidation dynamics in PSi right after preparation. Impedance Spectroscopy (IS) technique was applied on nanocrystalline silicon (NCSi) samples after submitting those surfaces to distinct phases of oxidation. Oxide phases were analyzed by Attenuated Total Reflectance in Fourier Transformed Infrared Spectroscopy (ATR–FTIR) and sessile drop Contact Angle (CA) technique. Nyquist plots were produced and studied by fitting the curves to a model containing a network of passive circuit elements. Results indicate a correlation between oxidation evolution (obtained from ATR and CA data) and the equivalent elements extracted from the impedance model, indicating a change in the surface chemistry of the semiconductor surface. This change is attributed to an increase of Si–O bonds in the original surface state. [ABSTRACT FROM AUTHOR]