We synthesize La0.9Ba1.1Ga0.5Mg0.05O4 - delta and then investigate electrical conduction properties, crystal structure, and nuclear and electron density distributions. From conductivity measurements, it is demonstrated that this sample exhibits higher conductivity than La0.9Ba1.1GaO4 - delta which is well-known as a protonic conductor. In addition, protonic conduction in La0.9Ba1.1Ga0.5Mg0.05O4 - delta confirmed by an HID isotope effect on the conductivity. In order to clarify proton site in the specimen, we measure neutron diffraction and then perform the Rietveld and maximum entropy method analyses. As a result, it is indicated that protons coordinate oxygens at O-3 site and form hydrogen bonds. It is also found that electronic states around oxygens are affected by the Ba and Mg substitutions. (C) 2013 Elsevier B.V. All rights reserved.