In this paper, processing and characterization of sheet, pellet, and membrane specimens based on the lithium aluminum germanium phosphate (LAGP) glass-ceramic system is reported. X-ray diffraction patterns exhibited the presence of Li1+xAlxGe2-x(PO4)(3) (x=0.5) as the primary phase. Increasing heat-treatment temperature from 850 to 950 degrees C led to the precipitation of an impurity phase, AlPO4, and a large increase in LAGP grain size. The highest total conductivity of the LAGP glass-ceramic material (5.08x10(-3) S/cm) at 27 degrees C was obtained by crystallizing the glass sheet at 850 degrees C for 12 h. The total conductivity of the specimen was in the range of 10(-3)-10(-1) S/cm in the 0-107 degrees C temperature range. The pelletized specimen prepared from the glass-ceramic powder and sintered at 850 degrees C for 9 h exhibited a slightly lower conductivity (4.62x10(-3) S/cm) at 27 degrees C. The membrane conductivity was above 10(-3) S/cm in the 37-107 degrees C temperature range. High grain boundary conductivity is apparent in the LAGP glass-ceramic materials. The impurity phases AlPO4 and Li2O were attributed to a characteristic nonlinearity in the Arrhenius plots and mediated the transport of the lithium ion, which is associated with a higher activation energy. (c) 2008 The Electrochemical Society. [DOI: 10.1149/1.2988731] All rights reserved.