The proton transfer reaction system View the MathML source has been investigated in both directions as a function of the mean relative kinetic energy, KEcm, between the reactants from 0.05 eV to 1 eV in a selected ion flow drift tube (SIFDT) experiment. The rate constant kf for the forward channel follows closely the calculated collisional limiting value, kc, showing a slightly negative energy dependence. The rate constant, kr, for the reverse channel, which is endoergic by 5.2 kcal mol−1, increases from kr = 2.3 × 10−12 cm3 s−1 at KEcm = 0.05 eV to kr = 2 × 10−10 cm3 s−1 at KEcm = 1 eV. This endoergic reaction is paralleled by an associative channel forming CH2OH+·H2O, which undergoes ligand switching with water molecules to produce H3O+·H2O, yielding a bond energy BE(CH2OH+−H2O) = 27.7 kcal mol−1 in agreement with previous data. The present results are important requisites to monitor the formaldehyde concentrations in air using proton transfer reactionmass spectrometry (PTR-MS).