By using hyper-Rayleigh scattering experiments and quantum-chemical calculations, we demonstrate that nonlinear optics can be used to probe unequivocally, within a non-destructive process, the multiple electronic states that are activated upon pH- and light-triggered transformations of the 4a-hydroxyflavylium ion. These results open new perspectives in the design of molecular-scale high-density optical memory. Remember this: Hyper-Raleigh scattering measurements provide evidence for large variations of the quadratic nonlinear optical responses to pH-triggered transformations of the 4a-hydroxyflavylium ion. By exploiting the capability of nonlinear optics to probe the electronic states without triggering uncontrolled photoconversions, the network of interconverting processes in this system might be a basis for the development of high-density multiple-storage optical memory.