We report the first fully analyzed high resolution spectrum of a carboxylic acid dimer in the gas phase. High resolution spectra in the region of the C-O stretch 1241.7-1250.7 cm(-1) have been recorded for (DCOOH)(2). The data could be fit within experimental uncertainty to a rigid rotor Watson S reduced Hamiltonian. The vibrational frequency of the C-O stretch in (DCOOH)(2) is determined to be 1244.8461 (2) cm(-1). Our spectra are the first direct experimental evidence for proton transfer tunneling in formic acid dimer (FAD), with FAD serving as a prototype for double hydrogen bonded organic complexes. Previous theoretical studies predicted proton transfer times covering a range of several orders of magnitude. Our measurements on (DCOOH)(2) established a proton transfer time of 5.8 ns [tunneling splitting of 0.00286(25) cm(-1)]. The proton transfer was found to accelerate upon vibrational excitation of the skeleton motion (the vibrational C-O mode), corresponding to a tunneling splitting of 0.00999(21) cm(-1). For this state the proton transfer time decreased to 1.7 ns, which corresponds to a vibrationally enhanced proton transfer mechanism. The moments of inertia are in agreement with a C-2h structure which indicates a high tunneling barrier. The observed spin statistical weights follow a description of formic acid dimer in the molecular symmetry group G(8), which is isomorphic to the point group D-2h. Our results suggest an out-of-plane contribution to the proton transfer mechanism for the otherwise planar dimer.