The infrared multiphoton dissociation (IR MPD) of formic acid molecules (HCOOH --> CO + H2O) with free electron laser radiation in the 3 mu m region (C-H stretch vibrational absorption band) and in the 6 mu m region (C=O stretch vibrational absorption band) has been achieved using the same laser. Results indicate that initial excitation of this dimer is more efficient for MPD then initial monomer excitation. Carbon isotope-selective MPD has been performed with an isotopic mixture, (HCOOH)-C-12 + (HCOOH)-C-13 (50% + 50%), at an irradiating wavelength of 5.56 mu m. For this mixture, isotopic selectivity s = k(12)/k(13) = 23 has been attained as a ratio of the rate constants of the (HCOOH)-C-12 MPD and the (HCOOH)-C-13 MPD, respectively. The isotopic selectivity for C-13 in the naturally occurring mixture (1.1% of C-13-containing molecules) s = k(13)/k(12) = 22 has been attained at an irradiating wavelength of 5.90 mu m.