The photochromic reaction dynamics of 6'-cyano-spiroindolinenaphthoxazine (6'CN-SNO) in solution is followed by femtosecond transient absorption spectroscopy. In addition to the formation of the merocyanine coloured form, we demonstrate the presence of a new metastable species with intense absorption bands in the 400-600 and 650-975 nm spectral regions that was not observed for the non-substituted parent molecule. The formation of this species parallels the photochromic reaction. The existence of this concurrent photophysical process can explain the 50% reduction in the photocoloration quantum yield of this molecule relative to that of the parent molecule. (C) 2004 Elsevier B.V. All rights reserved.