Picosecond time-resolved resonance Raman (TR3) spectroscopy was used to study the dynamics and structure of transient species involved in the photochromic process in solution of 1,3,3-trimethylspiro[indoline-naphthoxazine]. TR3 spectra recorded in n-hexane, at 10 ps and 50 ps time-delays were compared with those obtained on the ns-ms timescale and to the resonance Raman spectrum of a permanent opened form. These data and the results of a detailed ab initio vibrational analysis, have allowed us to determine the structure of the transoid isomer already present at 10 ps and to precise the photochromic mechanism.