We examined pore formation in thermally oxidized silicon wafers (SiO2 / Si) by means of swift heavy ion irradiation followed by chemical etching of latent track zones in SiO2 matrix. The samples were irradiated with 710 MeV Bi up to the fluences of (1-5)·108 and 5·10 10 cm −2 . Afterwards the targets were etched in the dilute solutions of hydrofluoric acid for various durations. Scanning electron microscopy was used to probe the processed samples. From the geometric parameters of the pores the etch rate Vt of the tracks and the etch rate V b of bulk a-SiO2 were estimated. The etching behaviour and morphology of the etched tracks has been found to change markedly with fluence. Mutual influence of tracks at their higher densities was analysed in terms of radiation-induced modifications of material around the ion path. It was shown that the morphology of etched tracks did not change after the annealing at 900• C for 30 min.