A transition state (TS) during Raman triggered oxidation of chloroform was directly observed by ultrafast spectroscopy using broadband visible sub-5 fs pulses. Changes in the molecular structures including the TSs along the reaction pathways were detected by the time-dependent instantaneous frequencies of vibrational modes. The Raman triggered oxidation of chloroform was found to be initiated by the excitation of vibrational levels through stimulated Raman processes involving the ground-state chloroform oxygen complex. (C) 2008 Published by Elsevier B.V.