Abstract Photodynamic properties of series of metal complexes having the general formula [M(diars) 2X 2]ClO 4 or BF 4 where M = Co 3+, Cr 3+, Rh 3+; X = Cl, Br, I, diars = o-phenylene bis(dimethylarsine) are studied. Photogeneration of singlet oxygen is monitored by both optical and EPR methods. In comparison with rose bengal ( ϕ( 1O 2) for RB = 0.76), singlet oxygen generating efficiencies of these complexes are determined. Rate of N, N-dimethyl-4-nitrosoaniline (RNO) bleaching is found to be retarded by specific 1O 2 quencher NaN 3, confirming the involvement of 1O 2 as an active intermediate. Photolysis of these complexes in the presence of spin trap 5,5-dimethyl-1-pyrroline- N-oxide (DMPO) generates 12-line EPR spectra, characteristic of O 2 − adduct. Photogeneration of O 2 − is also monitored by optical spectroscopy using superoxide dismutase (SOD) inhibitable cytochrome c reduction assay. The results indicate that the [Co(diars) 2Br 2]ClO 4 complex possesses high ability to generate reactive oxygen species (ROS). Both Type I and II paths are involved in the photosensitisation of the metal complexes. The antimicrobial activity of the complexes against selected bacteria is estimated. The relationship between the enzymatic production of ROS and antimicrobial activity of the complexes is examined and a good correlation between two factors is found. The [CoBr 2(diars) 2]ClO 4 complex investigated in this study effect photo cleavage of the plasmid DNA (pUC18).