Ionization and dissociation of 2- and 3-methyl cyclopentanones have been investigated in molecular beams by their irradiation with intense 394 and 788 nm laser fields with pulse duration of 90 fs and intensity of 3 x 10(13)-4 x 10(14) W/cm(2). The analysis of the resulting mass spectra allowed us to discern the effects of methyl substitution and its position on the outcome of ionization/dissociation processes induced by the intense femtosecond laser field. Generalized Keldysh-Faisal-Reiss (g-KFR) and ab initio G3(MP2, CCSD)//B3LYP/6-31G*/RRKM theoretical calculations helped to uncover the formation mechanism of major ionic fragments observed in the mass spectra including C5H10+, C4H6O+, C3H3O+, C3H4O+, C3Hx+, and C2Hx+. (C) 2013 Elsevier B.V. All rights reserved.