Abstract Fluorescence spectra of green fluorescent protein (GFP) at temperature 5 K under selective laser excitation in the lowest energy absorption band remain unstructured, indicating the absence of the zero phonon lines. This is interpreted as the result of charge transfer character of the electronic transition. The laser excitation through the absorption band at 499 nm creates the photoproduct band at 506 nm. The activation temperature (50 K) of the educt band recovery has been determined by temperature cycling. For the 471 nm (educt) and the 490 nm (photoproduct) band the activation temperature value is 130 K, indicating that the latter species are separated by substantially higher energetic barriers. The mechanism of the phototransformation has been ascribed to proton transfer between the adjacent energy minima along the direction of proton acceptor and donor sites of the chromophore.