Thermochromism was observed in sodium-loaded sodium-ion type zeolite FAU(I.25), Na,-Na85Al85Si107O384 with loading density x 8-10, in a temperature range of 300-600 K. With increasing x, the color of the pristine FAU powder changed dramatically from white to black via reddish brown at room temperature (RT). In the samples with intermediate loading densities, the reddish brown color reversibly changed to blue at high temperatures. Optical absorption measurements confirmed the thermochromism by observing a new absorption band around 1.7 eV at high temperatures. A possible mechanism related to the thermal conversion of cluster sites was discussed. @ 2006 Elsevier B.V. All rights reserved.