Time-resolved in situ electrochemical impedance spectroscopy measurements were performed in HF-electrolyte during the initial stages of nano- and mesoporous layer formation on p-Si and p(+)-Si electrodes respectively. A simple electrical equivalent circuit which involves a constant phase element to account for the distributed nature of the interfacial capacitance was chosen to model the impedance behaviour of the Si electrode. The experimental impedance spectra obtained were subjected to data validation and were then used to estimate the circuit parameters. Their time dependence is discussed in terms of the porous silicon growth mechanism.