This paper reports on a relationship between the conductivity and crystallization parameters (time and temperature) of a lithium aluminum germanium phosphate (LAGP) glass–ceramic. To study the relationship, a specific formulation of the LAGP [Li1+x Al x Ge2−x (PO4)3 (x =0.5)] glass was chosen. The glass was crystallized at different temperatures and times and it was characterized for microstructures and ionic conductivity. The highest total conductivity of the glass–ceramic material, 4.22×10−3 Scm−1 at 27°C, was obtained by crystallizing the glass at 825°C for 8h. The superionic conductivity in the specimens is attributed to the space charge-mediated effect resulting from the presence of the dielectric phase (AlPO4). The impurity phase AlPO4 imparted a characteristic non-linearity in the Arrhenius plots. A decrease in conductivity with an increase in crystallization temperature and time was observed. The observed conductivity was explained on the basis of the blocking effect resulting from an increased concentration or size of the impurity phase, AlPO4. The thermally unstable nature of the LAGP is reflected by the relationship between the conductivity and the crystallization parameters. Conductivity of the glass–ceramic is believed to be significantly affected by the space charge and blocking effects that originate from and relate to the crystallization temperature and time.