Thermal stability and devitrification of the xAgI (1 - x)AgPO3 system were studied by combining Differential Scanning Calorimetry (DSC), X-ray powder diffraction XRPD, and Ag-109 solid-state NMR. Particular attention was devoted to the glass-ceramic region (AgI-rich part of the system) because of its high bias to crystallization. By treating glass-ceramic samples at 300 degreesC, a more stable limiting glassy composition was found for x = 0.50. Total devitrification of glass-ceramic samples at 190 degreesC led to the formation of an intermediate compound with stoichiometry 2AgI.AgPO3, accompanied by AgI. The phase diagram of the AgI-rich part of the system is also proposed.