The synthesis, characterisation, photochemical and photophysical properties of ruthenium (II) and osmium (II) polypyridyl complexes containing 1,2,4-triazole ligands are described. These complexes were charactensed using HPLC, UV/vis spectroscopy, electrochemistry, NMR, fluorimetry, spectroelectrochemistry, laser flash photolysis and resonance Raman spectroscopy. Firstly, the photochemistry of a series of mononuclear Ru(ll) polypyridyl complexes has been investigated Evidence for the existence of a mononuclear photolysis intermediate during the irradiation of [Ru(bpy)2(4Mepytr)]2+ (where 4Mepytr = 4-methyl-3-(pyridin-2-yl)-1,2,4-triazole) was found Photoisomerisation from the N4-bound isomer to the N2-bound isomer of complexes containing the ligand Hpztr (where Hpztr = 3-(pyrazin-2-yl)-1,2,4-triazole) was noted, while the deprotonated species remained photostable. In the main section of the thesis, the photophysical and photochemical properties of a series of dinuclear Ru(ll) and Os(ll) polypyridyl complexes containing bridging 1,2,4-triazole ligands were examined. Investigation into the nature of the lowest emitting state was carried out using mixed-ligand dinuclear Ru(ll) polypyridyl complexes, containing the Hbpt ligand (where Hbpt = 3,5-bis- (pyridin-2-yl)-1,2,4-triazole). The nature of the lowest emitting state was found to depend on the polypyridyl ligand, whereas that of the reactive excited state (the 3MC excited state) depended on the asymmetric properties of the bridging ligand. The effect of replacing pyridine by the stronger 71-accepting pyrazine ring was examined using the ligand Hbpzt (where Hbpzt = 3,5-bis-(pyrazm-2-yl)-1,2,4- triazole) and it was found that the emitting state was bpy-based in the mononuclear species, whereas a switch-over to a pyrazme-based emission occurred upon formation of the dinuclear species. A combination of these properties were obtained using the Hppt ligand (where Hppt = 3-(pyrazin-2-yl)-5- (pyndm-2-yl)-1,2,4-triazole) and the photophysical properties investigated in detail. Its properties were compared with those obtained for the more symmetrical Hbpt and Hbpzt ligands. Again, a switch-over to a pyrazine-based emission occurred upon formation of the dinuclear species and upon protonation of the pyrazmebound monomer, whereas a bpy-based emission was observed for the pyridinebound mononuclear complex. Finally, the replacement of the strong a-donor 1,2,4-triazole ligands by the more strongly ji-accepting 1,2,4-triazine ligands was examined. The lowest emitting state was found to be pyridyltriazine-based and the effect of substituents on the physical properties of these complexes was evaluated.