A new dynamic approach to study chemisorbed species during the course of catalytic reaction was applied to the catalytic dehydration of formic acid over alumina and silica gel. Formic acid is chemisorbed on alumina surface and dissociates to formate ion and a proton. Formate ions on the alumina do not decompose directly to the reaction products and a fraction desorbs rapidly by exchange with formic acid. The rate of the reaction over the alumina is proportional to the number of protons from the dissociative adsorption of formic acid on the alumina surface, and also to the partial pressure of formic acid. Consequently, the decomposition proceeds between formic acid molecules and the surface protons supplied from the dissociative adsorption of formic acid, i.e., protons from formic acid behave as he reaction sites on the catalyst surface, while the formate ion on the catalyst surface does not behave as the reaction intermediate. For the formic acid dehydration on a silica gel, the adsorption of formic acid on silica gel is non-dissociative, and the rate of the decomposition is proportional to the pressure of formic acid and is independent of the amount of formic acid adsorbed. The silica gel treated with methanol to methoxylate its surface OH to OCH3 had no catalytic activity for the decomposition, which suggests that the surface OH is the reaction site for the decomposition. It is, accordingly, concluded that the decomposition of formic acid on the dehydrating metal oxide catalysts proceeds via the protonic sites on the surface, as with sulphuric acid in the liquid phase.