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The aim of this study is to measure the complex dielectric constant over a wide range of frequencies for ceramic powders often used as additives in polymer composites. The dielectric responses of alumina and boron nitride powders are presented. The variation of the dielectric constant in the studied frequency range (10-1 Hz to 10 kHz) showed that there is absorption of the water on the surface of the powder. Therefore, different treatments were applied: heating at 100°C for 48 hours and under vacuum at room temperature. A significant change on the value of the permittivity at low frequency has been observed after treatment. By comparing the results obtained before and after treatment, the dielectric response of the powder measured under vacuum showed stability over the studied frequency range at room temperature. The measured values of the permittivity of the powder were used to estimate those of the bulk material using a mixing law. This situation is discussed.