There has been increased interesting in melilite-type electrolytes, La1.54Sr0.46Ga3O7.27, where the conductivity is carried out through interstitial oxygen ions. This work presents the fabrication and characterization of Ga deficient and excessive melilites, i.e. La1.54Sr0.46Ga3+xO7.27-delta with x = -0.10, -0.05, 0.00, and 0.05. In addition, Ga is partially replaced with Al, In, Zn, and Ge to form doped melilites, La1.54Sr0.46Ga2.95M0.05O7.27-delta. X-ray diffraction analysis shows that the melilite structure is very sensitive to its composition, dopants, and even sintering temperatures. Composition deviation and element doping at a very small level could cause structure distortion. However, such distortion increases the conductivity at high temperatures due to the increase in activation energies for conduction. The results suggest that it is very effective to increase the electrolyte conductivity using Ga deficient and/or doped melilites. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.