Meso-tetrakisarylporphyrins have been perchlorinated and perbrominated at peripheral (β)-positions of the pyrrole rings by high-yielding procedures. These halogenated porphyrins show enhanced catalytic activity towards oxidation of alkanes and alkenes but are not stable when hydrogen peroxide is used as the oxygen donor. The reason for this instability appears to lie in excessive homolytic dissociation of H2O2 with consequent rapid attack at the porphyrin β-position.