The stability of the solid electrolyte Ag7I4PO4 in the presence of iodine under different conditions has been studied. When the iodine concentration is 1.7 X 10(-5) mol l(-1) in the gaseous environment, this compound can absorb up to 175 times its volume of gaseous I-2, measured at STP. Under 60% humidity, the sample weight increases linearly with time, reaching after 280 h a maximum ratio of 1.8 mol of iodine per mol of Ag7I4PO4. X-ray diffraction studies showed the formation of beta-AgI and gamma-AgI while the original compound is consumed. For a dry environment, we observed a continuous increase in sample weight with time up to 1.33 mol of iodine per mol of Ag7I4PO4 without saturation after 1460 h.