The excited-state intramolecular proton transfer of 2-(2'-hydroxyphenyl)benzothiazole was investigated with 30 fs temporal resolution. The proton takes 60 fs to arrive at its S-1 equilibrium position and the electron distribution changes on the same time scale. The molecule cannot adjust to the new structure of the H-chelate ring equally fast and starts to strongly vibrate in four low-frequency skeletal modes. This coherent wave packet motion is the cause of the experimentally observed strong signal oscillations that are only weakly damped despite the solvent environment.