This contribution summarizes some of the results from a study of the point defects and transport properties of nominally undoped and Ca-doped Ln(6)WO(12) (Ln=La, Nd, Gd and Er). The results presented are based on ac conductivity and impedance spectroscopy, EMF-transport number measurements and thermogravimetry. Moreover, isotope exchange has been applied to verify effects of protons. The conductivity of these materials is predominated by protons under wet atmospheres below 900 degrees C. The maximum in proton conductivity is observed for nominally undoped La6WO12 in the order of 3-5 center dot 10(-3)S/cm. At high temperatures, the materials exhibit n- and p-type electronic conduction under reducing and oxidizing conditions, respectively. Acceptor doping decreases the ionic conductivity for La6WO12 and Nd6WO12, whereas it increases the ionic conductivity for Gd6WO12 and Er6WO12. For undoped La6WO12 and Nd6WO12, it is speculated whether water dissolves at interstitial positions in the oxygen sublattice as protonic defects yielding the relatively high proton conductivity. The decrease in conductivity with the presence of Ca-doping may be a consequence of trapping of these protons. The enthalpy for formation of protonic defects becomes increasingly negative across the rare-earth series - from La to Er. (C) 2007 Elsevier B.V. All rights reserved.