Proton-bound clusters of formic acid and mixed clusters of formic acid and water evaporate neutral dimers of formic acid. This attribute is shared with liquid formic acid. Clusters are considered to bridge the gap between the gas phase and condensed phases. The evaporation of dimers begins already for hexamers. Collisionally activated dissociations indicate a structure in which a central ion (e.g. H3O+) is completely solvated by a ring of hydrogen-bonded formic acids. Ring closure and dimer evaporation begin far the same cluster sizes. Thermochemical considerations suggest that the formic acid dimer is evaporated in the open-chain configuration, rather than in the ring-closed one.