The ring-opening reaction of 1',3'-dihydro-1',3',3'-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2'-(2H)-indole] is investigated in two solvents, by probing the evolution of the vibrational absorption spectrum with 130 fs time-resolution. Competition between internal conversion and photochemistry is found to depend on the solvent. The internal conversion quantum yield is determined to be 0.63 in perdeuterated acetonitrile and 0.34 in tetrachloroethene. Based on spectral features and biexponential kinetics, the formation of an additional merocyanine isomer in tetrachloroethene is concluded. (C) 2003 Elsevier Science B.V. All rights reserved.