The catalytic test reaction of HCOOH decomposition on pure gamma-Al2O3 and doped (+ MgO (1% w/w)) was studied at temperatures 275-400degreesC. A double dehydrative and dehydrogenative, instead the thought single dehydrative, behaviour of gamma-Al2O3 was disclosed. The dehydration activity is higher for pure alumina and decreases with operation time while the dehydrogenation activity is higher for doped and increases with time at the higher but decreases at the lower temperatures showing an unusual temperature dependent activation and ageing. Doping and increase of temperature reduce dehydration selectivity. It decreases with operation time at higher but increases at lower temperatures. The Arrhenius kinetic parameters of reactions differ strongly. Activation energy of dehydration seems unaffected and pre-exponential factor slightly affected by catalyst kind and operation time. Parameters concerning dehydrogenation are affected slightly by catalyst kind and strongly by operation time. Based on an atomic scale structure analysis and results, two dehydration and one dehydrogenation coexisting mechanisms stemming from identical initial step of dissociative adsorption of HCOOH as H+ and HCOO- were suggested interpreting the results. The observed behaviour is due to changes of surface atomic spacing by doping Mg2+ and of mean distances of adsorptive pair sites with operation time caused by the second mechanism. (C) 2003 Elsevier B.V. All rights reserved.