In this study, we present a detailed investigation of the glass to crystal transformation in the Li1 + xAlxGe2 (-) xP3O12 (LAGP) system. Precursor glasses were synthesized following melt-quench and sol-gel procedures. The glass to crystal transition was studied employing X-ray diffraction and advanced solid state nuclear magnetic resonance (solid state NMR) techniques including Li-7-, Al-27-, P-31 NMR, Al-27{Li-7)- and Al-27{P-31)-REDOR-NMR as well as P-31{Al-27)-REAPDOR-NMR. Whereas Al-poor LAGP precursor glasses exhibit a direct glass-to-LAGP crystal transition, without any extended change in the AI content of the LAGP crystals and without the formation of side phases, for glasses richer in Al the crystallization of LAGP is accompanied by the formation of AlPO4, which for x = 1.0 is the first crystalline phase to precipitate. For these compositions, the first crystalline LAGP phase precipitating contains an Al content lower than the nominal x-value, however, with annealing, more AI is continuously incorporated into the LAGP structure, accompanied by a decrease in the relative amount of the AlPO4 phase. After surpassing a threshold temperature - which decreases from T = 900 degrees C for x = 0.4 to 720 degrees C for x = 0.8 - at which the LAGP phases reaches its highest Al content, close to the nominal composition, Al is gradually expelled from the LAGP structure, forming LGP and AlPO4. The findings enable the formation of virtually phase pure LAGP over an extended compositional range until x = 0.8. (C) 2016 Elsevier B.V. All rights reserved.