Calcium fluoride crystals doped with ErF3 and YbF3 have been grown using the vertical Bridgman method. The real and imaginary parts of the complex dielectric constant, epsilon' and epsilon", have been measured at five audio-frequencies (1-100 kHz) over the temperature range 150-300 K. From the observed relaxation processes the activation energy for dipole reorientation and the relaxation time constant,TO, have been determined. A comparison of our results with those obtained by other authors is also given. (c) 2005 Elsevier B.V. All rights reserved.