The DNA-binding modes and nuclease activity of methylpyridiniumyl/phenyl- hydroxamic acid porphyrin adducts (PHAs) have been studied. These compounds are derived from the tetracationic meso-tetrakis(N-methyl-4-pyridiniumyl)porphyrin (H2TMPyP-4) by replacing one or two pyridinium rings by a phenyl group. A hydroxamic acid function was connected by a polymethylenic chain of 3 to 5 carbon atoms to the meta or para position of the phenyl group. Different DNA-interaction modes were observed depending on the number of charges and hydroxamic acid functions. The nuclease activity in the presence of lanthanides was shown to be depending on the 1/r0 ratio and the nature of the lanthanide ion, and the optimum conditions are depicted after a systematic study. © 2011 World Scientific Publishing Company.