The effects of microwave irradiation of an alumina-supported 0.2 wt.% platinum catalyst are studied. In the case of the isomerization of hexanes, the selectivity in isomers is increased (by a factor 1.3 to 2) when the thermal treatment of the catalyst and/or the reaction are performed under the electromagnetic field. rhe isomer selectivity remains high even when such catalysts are subsequently tested under classical heating conditions. These permanent and profitable effects of the microwaves could be interpreted in terms of modifications of the geometric and/or electronic properties of the platinum particles. It is pointed out that the microwave irradiation could be a new and promising way for activating catalysts.