Compounds of LiGa1-xAlxGeO4 with 0 <= x <= 1 were examined using powder X-ray diffraction measurements and electrochemical impedance spectroscopy, to clarify their structures and Li-ion conductivity (sigma(Li)) as a function of x, and to compare with those for a wide variety of AB(2)O(4)-type oxides. The x = 0 and 0.25 samples were found to exhibit a phenacite structure with R (3) over bar space group, whereas the x >= 0.5 samples were found to exhibit an ordered phenacite structure with R3 space group due to the 1:1 cation order between Ga3+/Al3+ and Ge4+ ions. The sigma(Li) value for the x = 0 sample was 2.81(1) x 10(-5) S.cm(-1) at 973 K. Owing to the 1:1 cation order, both sigma(Li) and activation energy (E-a) were slightly improved at x = 1. The structural details of LiGa1-xAlxGeO4, possible conduction pathway of Li+ ions, and correlation between crystal structure and sigma(Li) (E-a) are presented. (C) 2016 The Electrochemical Society. All rights reserved.