We experimentally investigate the ionization and dissociation of two isomers, methyl acetate (CH3COOCH3) and ethyl formate (CH3CH2COOH), irradiated by strong laser pulses with a pulse duration of 7 fs. Strong parent ions are observed for both isomers. However, main fragmental ions are CH3CO+ and CH3+ for methyl acetate and CH3O+ and C2H4+ for ethyl formate. Through comparing with the single photon ionization mass spectra of these two isomers, we find that these fragments are produced by the direct dissociation of the parent ions in different excited electronic states. (C) 2008 Elsevier B. V. All rights reserved.