Ionization and fragmentation of formic acid dimers (HCOOH)(2) and (DCOOD)(2) by irradiation of femtosecond laser pulses (100 fs, 800 nm, similar to 1 x 10(14) W/cm(2)) were investigated by time-of-flight (TOF) mass spectrometry. In the TOF spectra, we observed fragment ions (HCOOH)H+, (HCOOH)- HCOO+, and H3O+, which were produced via the dissociative ionization of (HCOOH)(2). In addition, we found that the TOF signals of COO+, HCOO+, and HCOOH+ have small but clear side peaks, indicating fragmentation with large kinetic energy release caused by Coulomb explosion. On the basis of the momentum matching among pairs of the side peaks, a Coulomb explosion pathway of the dimer dication, (HCOOH)(2)(2+) -> HCOOH+ + HCOOH+, was identified with the total kinetic energy release of 3.6 eV. Quantum chemical calculations for energies of (HCOOH)(2)(2+) were also performed, and the kinetic energy release of the metastable dication was estimated to be 3.40 eV, showing good agreement with the observation. COO+ and HCOO+ signals with kinetic energies of 1.4 eV were tentatively assigned to be fragment ions through Coulomb explosion occurring after the elimination of a hydrogen atom or molecule from (HCOOH)(2)(2+). The present observation demonstrated that the formic acid dimer could be doubly ionized prior to hydrogen bond breaking by intense femtosecond laser fields.