This study reports the following new results for the photochemistry of the fluorescent protein Dreiklang, photoswitchable properties of which are due to the hydration/dehydration reaction of its GFP-like chromophore. Following the pathway from the level of the charge-transfer excited state we located the minimum energy conical intersection point the descent from which to the ground state potential energy surface allowed us to firmly characterize the reaction intermediate and the route to reaction product. This intermediate, termed X, was observed in the femtosecond studies but unjustly assigned to an adduct of the anionic chromophore and the water molecule . Consistently with the preliminary characterization of X in Ref. , its nature corresponds to the complex of the cationic chromophore and the anionic Tyr203 residue and the water molecule. We almost precisely reproduce the experimental absorption bands in Dreiklang, including the transient absorption band in X at 450 nm observed in femtosecond experiments and demonstrate that a low energy barrier separates this intermediate from the reaction product. Unlike previous proposals , we show here that the side chain of Glu222 does not serve as a proton shuttle in the hydration reaction; correspondingly, only the proton and the hydroxyl from the reactive water molecule occur in the hydrated chromophore.