Tunneling effects have been measured in the pulsed jet Fourier transform microwave spectra of two isotopologues of the benzoic acid−formic acid bimolecule. The tunneling splittings are originated by the concerted proton transfer of the two carboxylic hydrogens. From the values of these splittings for the OH−OH and OD−OD species, it has been possible to model/size the barrier to the concerted double proton transfer. SECTION: Spectroscopy, Photochemistry, and Excited States M olecular systems with two equivalent configurations that can interconvert upon proton exchange are particularly suitable to investigate the rate of such a process and the corresponding potential energy surface. High-resolution spec-troscopy, such as microwave (MW) spectroscopy, has provided considerable information on this kind of problem for a single proton transfer, for classical systems such as malonaldehyde (MA), 1 2-methyl-MA, 2 2-nitro-MA, 3 and tropolone. 4 In all cases, two equivalent structures with a C s symmetry are connected by proton exchange through a transition state with a C 2v symmetry. Extensive ab initio predictions for MA, including vibrational properties and a proton tunneling barrier of 13.8 kJ/ mol, were found to be consistent with experimental data. 5 A similar proton transfer takes place in pairs of carboxylic acids, but in this case, a double concerted proton transfer is required. The nature of such a kind of molecular system is depicted in Figure 1 for the bimolecule benzoic acid−formic acid (BA−FA). The two units are bound cooperatively together, since both act as proton donor and acceptor, forming a large eight-membered ring containing two hydrogen bonds. Such a kind of hydrogen bonding is the strongest one found within neutral species, with the monomers held together by more than 60 kJ/mol. Gilli et al. explain such a "strong" interaction in terms of a resonance-assisted hydrogen bond model. 6 Also for this kind of complex, MW spectroscopy can supply tunneling splittings. So far, interesting spectroscopic results have been obtained on the homodimers of carboxylic acids mainly with other high-resolution techniques. Among them, a rotationally resolved laser-induced fluorescence (LIF) investigation of the dimer of benzoic acid 7 allowed for the tunneling effects to be revealed. LIF methods generally require a chromophore group, restricting such studies to systems containing such a tag, so that the homodimers of simpler carboxylic acids have been investigated with other high-resolution methods such as femtosecond degenerate four-wave mixing and Raman spec-troscopy. These methods supplied information, such as tunneling splittings in the ground and vibrationally excited states, on