In aqueous solution (2 < pH < 8) the thermodynamically stable form of the 4’-methoxyflavylium ion (AH(+)) is its hydrated derivative trans-4’-methoxychalcone, C-t. The C-t compound shows a broad absorption band with lambda(max) = 350 nm. In acid medium, irradiation of C-t with near-UV light causes strong spectral changes with five isosbestic points and appearance of a very intense band in the visible region with maximum at 435 nm, corresponding to the AH(+) form. :It has been shown that irradiation of C-t causes a trans --> cis photoisomerization reaction (Phi = 0.04 at lambda(exc) = 365 nm), which is followed by 100% conversion of the cis-chalcone form (C-c) to the AH(+) ion. The AH(+) ion is photochemically inactive and thermally inert in acid medium (half-life of the back conversion at 25 degrees C in the dark is 815 days at pH 1.0 and 20 h at pH 4.3, respectively). At high temperature (>50 degrees C) and/or pH greater than or equal to 3, however, AH(+) can be quantitatively converted back to C-t (half-life of 15 min at pH 4.0 and 60 degrees C). Owing to this unique behavior, this represents a novel molecular system in which the color can be controlled by light and changes in temperature and/or pH. The ability to photochemically convert the stable and colorless C-t form to the kinetically inert and colored AH(+) form, and the possibility to reconvert AH(+) to C-t at high temperature or by a pH jump make the system well-suited as the basis for an optical memory device with multiple storage and nondestructive readout capacity through a write-lock-read-unlock-erase cycle.