The structural transformations of flavylium ion in aqueous C t form. Such a back reaction to C t can be totally prevented if the irradiated solution is submitted to a pH jump to pH = solutions caused by pH jumps and photoexcitation have been investigated. At pH Ͻ 1, the stable form is the colored 1, which transforms the photoproducts into the stable AH + species. In basic solution, two more species were detected, cationic species (AH + ). By increasing pH, the concentration of AH + decreases and, at pH = 5, this form is no longer namely the anionic forms C c -and C t -of the cis and trans chalcone. C t -is a stable, not photosensitive and luminescent present. The species obtained immediately after a pH jump undergo transformation processes with pH-dependent rate species, whereas C c -is not stable, being converted into C t in the dark. The photochemical and pH-induced constants. At pH = 5.2 and 20 °C, the final product is the uncolored trans-chalcone (C t ). This form can be transformed transformations of the flavylium cation in the pH range 0-11 can be taken as a basis to design write-lock-read-unlock-by light excitation into the cis-chalcone (C c ) isomer, which is in equilibrium with the hemiacetal form (B 2 ). This mixture is erase cycles for an optical molecular-level memory with multiple readout capacity. relatively inert due to the existence of a kinetic barrier that slows down the back thermal isomerization of C c to the stable complex structural transformations as represented in order to extend the knowledge of the reversible transform- [a] Departamento de Quı ´mica, ation processes that take place in this family of compounds, Centro de Quı ´mica Fina e Biotecnologia, we have performed an investigation on the behaviour of the