Flavylium salts exhibited a reversible thermochromism in an agar-gel matrix. Thus, by the temperature change from -3 to 50 degrees C, the absorption band around 600 nm of 7-diethylamino-4'-dimethylaminoflavylium perchlorate increased by a factor of ca. 1.5, while the band around 520 nm substantially decreased. Upon repeated cycles of the color change no significant fatigues were observed. The thermochromic behaviors were tentatively rationalized to the dye aggregation phenomenon enhanced by the sol-gel matrix.