NASICON-type Li1+xAlxGe2-x(PO4)(3) solid state lithium ionic conductors were synthesized by a sol-gel method using citric acid and ethylene glycol. The obtained precursors were sintered at various temperatures and the NASICON-type single phase was observed in a range of x = 0-0.6. The highest electrical conductivity was obtained for Li1.4Al0.4Ge1.6(PO4)(3) sintered at 900 degrees C for 11 h in air. The total conductivity was 1.22x10(-3) S cm(-1) at 25 degrees C, and the bulk and grain boundary conductivities were estimated by impedance profile analysis to be 1.70x10(-3) and 4.30x10(-3) S cm(-1), respectively. Sintered pellets of Li1.4Al0.4Ge1.6(PO4)(3) were immersed in distilled water, saturated LiCl aqueous solution, and a saturated LiCl and LiOH aqueous solution at 50 degrees C for one week; X-ray diffraction patterns of these samples dried at 220 degrees C under vacuum showed no significant change from that of the pristine sample. The electrical conductivity of Li1.4Al0.4Ge1.6(PO4)(3) was decreased to 1.4x10(-4) S cm(-1) at 25 degrees C by immersion in distilled water, while immersion in the saturated LiCl aqueous solution increased the conductivity to 4.95x10(-3) S cm(-1) at 25 degrees C. Li1.4Al0.4Ge1.6(PO4)(3) was stable in the saturated LiCl and LiOH aqueous solution. Li1.4Al0.4Ge1.6(PO4)(3) was unstable in contact with Li metal and Li-In alloy, but was stable in contact with Li7Ti5O12. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.080207jes] All rights reserved.