Solid ionic conductors composed of Li1.4Al0.4Ti1.6 - xGex(PO4)(3) (x = 0-1.0) with the NASICON-type structure were synthesized with a precursor prepared using the sol-gel method. The electrical conductivity was examined as a function of x in Li1.4Al0.4Ti1.6 - xGex(PO4)(3) at various sintering temperatures and for various sintering periods. The highest electrical conductivity was obtained for Li1.4Al0.4Ti1.6 - xGex(PO4)(3) sintered at 900 degrees C for 11 h in air. The total and bulk conductivities of the sintered pellet were 1.29 x 10(-3) and 2.35 x 10(-3) S cm(-1) at 25 degrees C, respectively. The grain boundary resistance of Li1.4Al0.4Ti1.6 - xGex(PO4)(3) was significantly increased to 552 Omega cm(2) from 12.2 Omega cm(2) by immersion in distilled water at 50 degrees C for one week, whereas the bulk resistance was not increased. However, no significant increase of the bulk and grain boundary resistance was observed after immersion in a saturated aqueous solution of LiOH and LiCl. (C) 2013 Elsevier B.V. All rights reserved.