Process for regenerating or activating a catalyst for isomerizing n-paraffins, containing at least one group VIII metal supported on an acid mordenite, wherein, after having lost at least a part of its initial activity, said catalyst is treated in a first step with an oxygen-containing gas at a temperature lower than about 550.degree. C., so as to remove the major part of the catalyst coke content, and in a second step the resultant product from the first step is oxychlorinated to a temperature of about 200.degree.-500.degree. C. by means of a gas mixture containing oxygen, water and chlorine or at least one chlorinated compound, said chlorine or chlorinated compound being used in a total amount of 0.5-10% by weight, calculated as chlorine, in proportion to the mordenite weight.