The electrodissolution of Ti and p-Si electrodes in acidic fluoride solutions has been investigated by electrochemical impedance spectroscopy as a function of potential E, fluoride concentration, pH and angular speed of the electrode, with the aim of characterising the oxide layers covering both materials. The impedance diagrams have been analysed to obtain the low frequency capacitance C-lf the high frequency capacitance C-hf and the product RhfI (where R-hf is the high frequency resistance and I the steady-state current). In the potential domain where the oxide thickness x is proportional to E, the formation ratio dx/dE has been computed from C-lf, C-hf and RhfI. The calculated values have been found to be in good mutual agreement and close to the ones found in the literature.