In this work we applied FTIR vibrational spectroscopy in Attenuated Total Reflectance mode, to observe the transient species, in an acetone solution at very low weight concentration, due to the molecular transformation from spiropyran to merocyanine form, in four different spiropyran types. In UWIS spectroscopy a new peak generation in the visible region was observe. In ATR, the peak shift from 1487 cm-1 to 1510 cm-1 and two new peaks generation at 1568 cm-1 and 1595 cm-1 are associated to the ring. Also, due to the C-O break down bonding when the molecular transformation was done, the peak associated to the O-C-N bond diminished in intensity and the new peaks generations between 900 to 1000 cm-1 , are associated to this molecular transformation. © 2008 American Institute of Physics.