Optical transmittances of the hydrated Ag-4A zeolite were measured under vacuum at elevated temperatures. Optical transmittance measurements reveal that there are two stages of the creation of the Ag cluster under 80 °C. The evacuation below 50 °C induces the absorption above 2.2 eV. The evacuation above 60 °C brings about the new peak at 2.8 eV which peak is origin of yellow color. [DOI: 10.1380/ejssnt.2005.270]