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 λ(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 (Φ = 0.04 at λ(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 °C in the dark is 815 days at pH 1.0 and 20 h at pH 4.3, respectively). At high temperature (>50 °C) and/or pH ≤3, however, AH+ can be quantitatively converted back to C(t) (half-life of 15 min at pH 4.0 and 60 °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.