Thermal decomposition of HCOOH has been studied from 1280 to 2030 K over a broad range of initial concentrations (0.07 to 1.6% in Ar) in a shock tube using a stabilized cw CO laser to monitor CO formation. The observed CO concentration profiles over the whole range of temperature and pressureemployed could be satisfactorily accounted for by the following two-channel molecular decomposition mechanism: (HCOOH + M over(→, 1) CO + H2 O + M; HCOOH + M over(→, 2) CO2 + H2 O + M)89-1 with the following apparent second order rate constants in the pressure range 0.75-2.8 atm. (k1 = 2.3 x 1015 exp (- 50, 000 ± 1700 / RT) c c mole- 1 sec- 1; k2 = 1.5 x 1016 exp (- 57, 000 ± 2800 / RT) c c mole- 1 sec- 1) The values of these two rate constants were evaluated by the non-linear least squares fitting of the observed CO concentration profiles. The possibility of an alternative decomposition route involving the initial production of H atoms, instead of H2, has also been considered. However, the rate constant evaluated from the kintic modeling of the CO formed at high temperatures cannot satisfactorily account for that observed at lower temperatures.