The performance at 300 degrees C of Li2.9Nb0.9M0.1O4, M = W or Mo, as a solid electrolyte is compared to that reported in 2004 by McLaren et al. for Li2.9Ni0.05NbO4. The Li+ conductivity and activation energy were found to be nearly identical for M = W. Ni in the Li array does not impede Li+ mobility more than aliovalent substitution for Nb, and W(VI) is not reduced before Nb(V). Substitution of Nb(V) by Mo(VI) was not as advantageous. (C) 2012 Elsevier Ltd. All rights reserved.