Nitro-nitrito photoisomerisation is investigated in solid samples and solutions of trans-[Ru(py)(4)(NO(2))(2)]. Using light of wavelength 325 nm 50% of the N-bound Ru-NO(2) ligands can be switched to the O-bound Ru-ONO configuration (nitrito-N to nitrito-O isomerisation) at temperatures below T = 250 K in solids. The population of the isomeric configurations is determined with infrared spectroscopy from the decrease of the area of the nu(NO) stretching and delta(NO) deformation modes. In a frozen methanol-ethanol solution nearly 100% can be converted to the nitrito-O configuration. Upon heating above T = 250 K the Ru-NO(2) configuration is restored. The nitrito-O Ru-ONO configuration can be partially transferred back to the nitrito-N configuration by irradiation with light in the spectral range 405-442 nm. Using absorption spectroscopy on a frozen methanol-ethanol solution, two new bands at 447 and 380 nm are observed in the nitrito-O configuration compared to one at 334 nm of the nitrito-N ground state configuration. The photoconversion is initiated by the metal-to ligand charge transfer transition Ru(d) -->pi*(NO(2),py) as shown by the calculated partial density of states using Density Functional Theory. The calculations yield also the structure of the nitrito-N and nitrito-O isomer as well as the corresponding vibrational densities. The experimental structure of the ground state is determined using powder diffraction.