The dielectric properties of NaF doped and undoped kaolinite were measured as a function of frequency in the range 40 kHz to 50 MHz. The epsilon, tan delta and sigma(ac) values are given as a function of firing temperature and NaF concentration. The addition of NaF increased the dielectric constant of pure kaolinite. However, the dielectric constant and ac conductivity of mullite obtained by the addition of NaF to kaolinite decreased from the value for pure kaolinite. Furthermore, the addition of NaF reduced the formation temperature of mullite from 1200 to 1000 degrees C. (c) 2006 Elsevier Ltd and Techna Group S.r.l. All rights reserved.