The efficiency of ruthenium complexes for photosensitizing DNA damage depends on the oxidizing character of their ligands. Here we report on the difference in behavior of tris(2.2'-bipyrazyl)ruthenium(II) (Ru[bpz]3(2+)), tris(2,2'-bipyridyl)ruthenium(II) (Ru[bipy]3(2+)) and cis-dichlorobis (2,2'-bipyrazyl)ruthenium(II) (Ru[bpz]2Cl2). Upon irradiation at 436 nm, Ru(bpz)3 (2+) was far less stable than Ru(bipy)3(2+). Ru(bpz)3(2+) in phosphate buffer containing NaCl undergoes a photoanation reaction leading to the formation of Ru(bpz)2Cl2, as previously reported also in organic media. In the presence of phage phi X174 DNA, Ru(bpz)3(2+) photosensitized the formation of single strand breaks with an efficiency that was, at the beginning of irradiation, similar to that of Ru(bipy)3(2+). After 8 min of irradiation, the cleavage efficiency of Ru(bpz)3(2+) reached a plateau that may correspond to its photode composition. For the same conditions, Ru(bpz)2Cl2 did not induce DNA breakage. Scavenging experiments showed that, in the presence of oxygen, DNA cleavage induced by Ru(bpz)3(2+) partly resulted from the formation of singlet oxygen and hydroxyl radical while in the absence of oxygen an additional mechanism involving electron transfer between the excited state of the ruthenium complex and DNA is proposed. The ICP measurement showed that Ru(bpz)3(2+) and Ru(bpz)2Cl2 gave rise to covalent binding onto DNA in contrast with Ru(bipy)3(2+), which did not bind to DNA under the experimental conditions. The results are discussed with regard to the potential use of these photosensitizers in phototherapy.