The structure and chemical composition of porous silicon (PS) have been investigated as the function of hydrofluoric acid (HF) concentration in solution at electrochemical formation by scanning electron microscopy (SEM) and infra-red (IR) spectroscopy. The PS layers were fabricated in concentrated (similar to 80%) and in diluted (similar to 20%) HF-solutions whereas current density remains constant (similar to 30 mA/cm(2)). It was found that the polymerized mesoporous silicon films (meso-PS) have been formed at high HF concentration whereas the macroporous silicon layers (macro-PS) were grown at low HF concentration. A FTIR spectrum measured in a wide range (400 divided by 4000 cm(-1)) exhibit the polymerized hydride films (SiH)(n) on the meso-PS surface, while the silicon oxide was defined on macro-PS surface. This is in accordance with secondary ion mass spectrometry (SIMS) studies. The photoluminescence (PL) studies show that PL peak position corresponds to 655 nm in the case of meso-PS. In contrast, PL spectrum of macro-PS was shifted to shorter wavelength (peak around 600 nm) with a weak intensity. Basic parameters of the PS layers have also been found: the mean pore sizes (d=2-8 nm for meso-PS from the low-temperature nitrogen adsorption, Brunauer, Emmet, Teller (BET) method and d=500-3000 nm for the macro-PS from SEM imaging), a porosity (p=60% for meso-PS and p=86% for macro-PS from gravimetric data), the coefficient of refraction (n=1.75 for meso-PS from IR spectrum), the PS thickness h=5 divided by 20 mu m from profilometric measurements and IR spectrum). (c) 2005 Elsevier B.V. All rights reserved.