Thermal reactions of (HCOOHH+ (n = 1-3) with a series of base molecules B were studied using a selected ion flow tube. Reactions observed include association, ligand switching, and proton transfer and occur at gas collision rates for all the bases having proton affinities (PAs) higher than formic acid. Branching ratios between the various reaction products depend on the proton affinity of the neutral reagent, but specific chemical reactions were observed for different neutrals having similar PAs. Thermochemical calculations suggest the evaporation of the formic acid neutral dimer in some ion/molecule reactions of (HCOOHH+. The proton binding abilities of the two groups-C=O and OH-in formic acid play a role in some of the end products which have been observed. The reaction of (HCOOHH+ with acetonitrile leads to a mixed trimer in which protonated formic acid is the core ion, while reaction with ammonia leads to a mixed trimer in which protonated ammonia is the core ion.